Verb. To investigate so as to discover the truth about a person or thing. Until Wes Davis suggested this to me, I had no idea it was a Britishism. It is. (Wes says he first heard it on BBC programs that aired here in the 1970s.) It seems to have originated in the sort of British noir slang sus, short for “suspect” (both the noun and verb). The OED cites D. Webb’s 1953 novel Crime Is My Business: “He turned to Hodge and said, ‘Who’s sussed for this job?'” This form first shows up in a 1966 article in Queen: “Youth susses things out on its own.” Google Ngram indicates that the expression turned up in the U.S. circa 1990 and has been steadily increasing in popularity ever since.
Fifty-nine to 36, 125 to 119, 5 to 2, 4 to 0; scores are all so obvious and pure — too damned obvious and pure for those of us inclined to suss out subtle meanings and unseen truths. (Kurt Andersen, Time, January 17, 1994)/We’ve taken a close look at all the pilots in production, talked to insiders, and read the scripts to suss out the twenty most exciting and intriguing ones, and then handicapped their chances of success. (New York Magazine, April 7, 2011)
notoneoffbritishisms.com 
At some point the historical method will get you into a jam here (“and things will go pear-shaped”) because the great central trunk and roots of American English are of course nothing but Britishisms. Good luck!
George, I am counting on you to let me know when I get to that point!
I learned the English slang word (now colloquialism?) from a song lyric years ago:
Who, The
Tommy (1969)
We’re Not Gonna Take It
. . .
Hey you getting drunk, so sorry!
I’ve got you sussed.
Hey you smoking Mother Nature!
This is a bust!
(From http://www.purelyrics.com/index.php?lyrics=rsweigyf)
I feel that I must’ve learnt a lot of british-english from music lyrics growing up without realizing it, sponges that young brains are. I’m certain there are any number of other songs, but the Jam’s “Butterfly Collector” (“You use your senses to suss out this week’s climber”) and Stiff Little Fingers (N. Ireland) “Suspect Device”, where they played with the word, suss, a bit, didn’t escape my notice at age 11. I can’t remember not knowing what the word meant. My parents definitely used the word as did/do I. Although the use seemed obviously higher in the UK, I’m surprised that it’s a britishism. Without knowing it, it could have easily been one of those (observed) generational words usages (US) that gets watered down between generations. Sort of like (I hear) “you lot” has in the UK.
Recently, ‘suss out’ was used repeatedly in an episode of a law & order-type TV program called “Justified” set in eastern Kentucky within the context of fa conversation about ‘rent boys’.
I vividly remember encountering “suss” for the first non-British time in one of the very first Tina Brown-edited New Yorkers.
It was in an article about Daniel Day-Lewis in October 1992, in which someone was quoted as a saying, “I sussed that in a sense it would bring him back to a life and a reality that would give him a focus and a structure.” The word was so unfamiliar at the time that someone wrote a letter to the New Yorker suggesting it was a typo: “I assume that she actually said ‘supposed’ or ‘suggested.'” “The Editors” replied: “‘Suss’ means ‘suspect’ or ‘figure out.’ It’s one of those Britishisms, like ‘cess’ or ‘fug’ which probably came from centuries of poor enunciation.”
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The OED drew a blank, but Google Books provides us with the earliest instance as 1961 in The Synonym Finder, 1961, by Jerome Irving Rodale, giving it as a synonym for “investigate.” There might be an earlier instance, but Google doesn’t provide the publication information, and when it comes to periodicals, often provides the incorrect date.
Then there’s the possibility that it entered BrE from French, specifically the imperfect subjunctive of ‘savoir’ (to know): je susse
I watch a lot of British television and films, but the only time ive heard it used is in the long-running detective show “A touch of frost.” Where ive also encountered other words or phrases such as “do a runner,” “bunk,” “slag each other off,” and “naf” — the last two I still don’t know the meaning.
Well, regarding an early poster, I don’t think all American slang, for instance, are Brittishisms.
“slag each other off” means an exchange of derogatory, critical remarks. “naf” is something that’s no good – cheap or tasteless, sometimes both. You can also telll someone to “naf off” – shift your useless carcass out of the way. Hope this helps.
And then of course there were (are?) the “suss laws”.
It’s actually an Irishism. Coined by playwright Seán O’Casey, for the colourful character of Joxer in ‘Juno and the Paycock’ in the early 1920s. O’Casey might well have been thinking of the word ‘suspect’.. who knows? But this is his baby! 😉
Is it really? That is interesting. I’m a New Zealander, 40y/o, and I’ve always thought it was just us and Australians that used this term. We use “I will suss it out” to mean that we will organise something or work something out.
There seems to be a second meaning too: an abbreviation of “suspect” or “suspicious”. For example: “These statistics seem a bit sus”. In all honesty, I’m not sure of the spelling in popular usage – one ‘s’ or two – but really it should be one.