New York Times theater critic Ben Brantley is a frequent user of Not One-Off Britishisms, presumably having picked them up during all the time he spends in London going to plays. In the third paragraph of a review last week of a New York production of Gogol’s The Government Inspector, Brantley referred to an actor “called” (instead of “named”) Michael Urie, which led me to turn on my NOOBs-dar. And sure enough one came along just a few paragraphs later:
Any suspense in the plot as to do with anticipating when, or if, the townsfolk will twig onto Ivan’s true identity…
“Twig onto” was unfamiliar to me, but it seemed to have a distinct British feel. Green’s Dictionary of Slang has a long entry for the verb, with definitions and citations dating from the eighteenth century for four different (similar) meanings: “to observe, to watch”; “to understand, to work out”; “to recognize, to expose”; and “to catch sight of, to become aware of.” Interestingly, the dozens of citation almost all use “twig” alone, rather than followed by “onto,” as Brantley has it. For example, from Alan Sillitoe’s 1958 novel, Saturday Night and Sunday Morning: “Brenda had never the agile mind to twig that he was whiling the days between times away with her sister.”
Green’s suggests the word may have been derived from “twick,” meaning “to jerk,” but Stan Carey has (for me) more convincingly argued for a derivation from the Irish “tuig,” meaning “understand.” The argument is bolstered by the fact that the first citation in Green’s (it’s the “observe, watch” meaning) is from the 1754 play The Brave Irishman, by the (Irish) Thomas Sheridan: “Twig his boots.”
Back to the “twig onto” matter, a search for “twigged onto” on Google News yields a mere sixteen hits, from an intriguing variety of locations: the U.S., U.K., Australia, New Zealand, and India. But “twigged that” pops up 1,030 times, the overwhelming majority from the U.K. The takeway is that Brantley got it wrong, and should give some thought to the proposition that if you’re going to use a Britishism, you should use it correctly.
notoneoffbritishisms.com 
I have never encountered ‘twigged onto’ before. In the UK to twig is to cotton-on or for the penny to drop. e.g.
“I thought she had put on weight but I hadn’t twigged until I saw the pregnancy test kit in the bin.”
‘twig on’ seems like a combination of ‘twig’ and ‘catch on’, which has a similar meaning.
Twig onto or twig on to? I’d go for the latter version — but then, I’m not a native speaker.
Ten Google News hits for “twig on to.” There is a longterm trend in English to use “onto” instead of “on to.”
Ten hits… that’s not all that many less [of fewer, your choice] than the sixteen hits for “twigged onto” 😉
The point is that they’re both very small compared to “twigged that.” Sort of intermediate popularity is “twigged to,” with 341 hits.
As a Canadian I use the expression a lot but I and everyone else I know who uses it would always say, “twigged to” rather than “twigged onto”. My Scottish mother uses “twig” on its own in the sense of finally apprehending something and would ask me, as a teenager, “Do you twig the problem you cause the rest of us when you sleep until noon”. I didn’t then and I still don’t!
I think ”to” is sometimes added when twig is used in the past tense. For example, ”He twigged to the fact that she was extracting the urine.”
But, ”to the fact” is really superfluous to the meaning and merely adds emphasis. I think that is what Ben brantley was trying to do, but he was unsure of the usage.
You don’t say Twig onto. It’s twig. Do you twig? Did you twig what he was up to? The onto is an abomination. Brantley may be thinking it is used like catch on to. Twig to? No, never. It would be like saying Do you understand to? Did you notice to? The Irish Gaelic verb tuig (to understand) sounds (loosely) like tig or t(h)ig – not a million miles from twig. There is also the slang Do you dig? meaning Do you get it? Sounds not unlike An dtuigeann tu?(loosely On diggan thoo?), meaning Do you understand? I wonder if that’s where dig = understand comes from. To add to the mix, an older Scottish (not Scottish Gaelic but Lowland Scots) spelling of twig is tuig. The plot twickens. Twickens – a connection with tweak? And there’s a place near here called (Oops! named) Twickenham which probably has nothing to do with it.
The idea of English “twig” (= understand, catch on) coming from Irish “tuig” is superficially attractive. That is to say: it pleases the eye. But there’s a big problem in that “tuig” actually sounds much more like “tig” (or even, “tick”). The “u” in the Irish word is not itself pronounced: it’s there simply to indicate that the t is velarized (pronounced with the back of the tongue raised). I’m reminded of a time when I was learning (attempting to learn!) Irish and a child in the family I was staying with asked me how you could tell that Irish watches were very clever. He answered the riddle for me by holding his watch to my ear and asking me couldn’t I hear it saying “tuig, tuig, tuig”?
“twig on” from Victoria Wood As Seen on TV, 1985:
“She’s someone to notice then. Our Jack wouldn’t. I tell you this, Liberace could come through in a long-line bra and our Jack wouldn’t twig on. First night of our honeymoon, I were lying in bed, he were making a hutch!”
It’s straightforward: “twig” means to catch on, so you don’t want an additional “on” or “onto”. You either twig something, or just twig, i.e. get wise, understand the statement or situation.
On Oct 16, 2018. on the episode “Other Worlds” of the PBS series “The Great American Read”, literary critic Marie Arana uses “twig to” in the sense of ‘understand’ or ‘perceive’ at ca. minute 43:40: “I think the novel of spiritual attainment, that razor’s edge story, traveling that fine line to find out what you believe in, is something that we all twig to as readers.” She was talking about the novel The Shack.
As a moderately old Brit (originally from South Wales) I found Brantley’s usage almost perfect. The only problem was omitting the space between the phrasal verb “to twig on” (as used by Victoria Wood above) and the indirect object, “to Ivan’s true identity”. It seems very unnatural to merge the last word of one phrase with the first word of the next – it disrespects the tree-structure of the sentence. (Alternatively, it requires you to think there are two different phrasal verbs, intransitive “to twig on” and transitive “to twig onto”.)
To me, “to twig” with no “on” sounds strange. I would hear it either as an incomplete version of “to twig on”, or as a totally different verb that is not in my active vocabulary. The latter, as used by Lesley’s Scottish mother, just means to understand, while “to twig on” means to COME to understand, (with an implication that it was done rather belatedly, if the sentence is not negative.)
Of course, that’s just my idiolect, so your mileage will quite likely vary.
OK, I’ve done a bit of searching and it looks like my idiolect is now in a smallish minority, despite my strong impression that people always used to put in the “on”, and that leaving it out was a lazy modern trend.
I’ve also found a couple of examples where “to twig on” takes a direct object, and they both seem fine to me. The grammar of “twig” is surprisingly complicated.
1) From an American website: “Following a conversation with Vinny, Thomas twigged on the idea to move up his wedding date to Zoe, thereby causing Douglas to panic and upping the pressure on Hope to intervene and agree to marry Thomas herself.” (https://soaps.sheknows.com/the-bold-and-the-beautiful/news/561050/bb-spoiler-speculation-thomas-scheme-unravels-hope-shocks-guests-wedding/) As “the idea” is a plan of action that the person creates anew, not a pre-existing fact that they realise or understand, “twigged on the idea” is perfect. “Twigged on to the idea” and “twigged the idea” sound wrong to me.
2) From Canada: “I wondered [why] all those cars were coming, but then all of a sudden, it twigged on me!” (https://www.cbc.ca/news/canada/manitoba/winnipeggers-hold-easter-drivebys-for-grandparents-isolated-during-covid-19-1.5530478) Here “I twigged on to it” has become “it twigged on me”, and it actually sounds more natural, at least in this pronoun-heavy sentence.
For me (English East Midlands) ‘to twig’ is ‘to realise’, not ‘to understand’. But it’s unusual in this neck of the woods; I associate the verb more with older speakers from southeast England, counties south of London particularly.
‘Then he realised.’ seems unfinished, hence ‘Then he twigged.’ or ‘Then the penny dropped.’ The most common short expression must surely be ‘He never twigged.’ (I, she, they).
And all this time I though “twig” was a Canadianism. I remember flying from Toronto, probably with a connecting flight in Chicago or somewhere, to a conference somewhere else in the U.S. I fell into conversation with my seat mate, a pharmaceutical sales rep. When I used the word “twig,” he stopped me and asked about it – and then made a note. I couldn’t remember if “twig onto” is correct usage or if it’s just “twig.” When I got home I asked a born-Canadian friend who said it’s just “twig” – no “onto.”