I was surprised to find that I have never covered this term for a stupid or foolish person, given that I have done wazzock and shitgibbon, and that British insults are much in the air. The OED describes “numpty” as originating in Scotland and gives this possible etymology: “Origin uncertain; perhaps an alteration of numps n. or numbskull n., with ending perhaps remodelled after humpty-dumpty n.” The first citation is from 1985 (‘They are a pair of turkeys,’ he said. ‘Numpties, the both of them.’–P. Firth, The Great Pervader), and more recent ones show a migration to England.
Neither the OED nor Green’s Dictionary of Slang show U.S. use, but the term did appear in the New York Times in 2009, when Dallas-born novelist Bill Cotter said:
In the mid-’80s, Boston’s Kenmore Square, where part of [his novel Fever Chart] is set, was home to three-card-monte men, ordinary punks, beer-devastated Red Sox bleacher-seat numpties, the Guardian Angel menace, and the only music venue worth visiting in that fourth-rate city, the Rat, a black basement often populated with bloody-nosed hardcore girls swinging tiny fists of stone.
More recently, I’ve started to see it over on Twitter, inevitably regarding the most frequent subject of all these insults, the U.S. president. As I’ve noted previously, the Tweetdeck application lets you filter tweets geographically. Here are some recent ones that use “numpty” and were posted within 200 kilometers of New York City:
notoneoffbritishisms.com 
“Numpty” came up in the comments to the “wazzock” post. I wonder if the people using the word when responding to Trump’s tweets are influenced by the presence of the “ump” letter sequence in both words. There seems to be an implicit reference to the name Donald Numpty at play.
That was my first thought too – they’ve got the “ump!”
Thanks for letting me know it was mentioned previously. I guess the “Search” function doesn’t include comments, which is too bad.
“Numpty” rhymes with “Trumpty Dumpty,” who appears in some memes sitting on his wall.
I grew up with “numpty” as a term used for stupid people who had no consideration for others so it probably makes sense that it is being used in the States primarily in connection to your president. I think it was the only non-profane insult regularly used by Jamie McDonald in “The Thick of it”.
Also in use variously in the UK in insult form, can be used as mild and fond when talking to friends, or as insults thrown across a street accompanied by hand gestures and much puff-chesting nonsense. Behold:
‘Muppet’, ‘plebneb’, ‘chump’, ‘nugget’, ‘prat’, ‘plank’, ‘dapper’ (as in, ‘you dapper!’ rather than ‘what a dapper (distinguished) fellow he is, heyho!’), ‘plonker’, ‘prannock’ (variant of ‘prat’), ‘spam’, ‘knob/nob’, ‘doughnut/donut’, ‘spoon’. I’m sure there are many more. Also no doubt some are used in Australia/New Zealand. I believe ‘drongo’ is one of their finest. 🙂 I’d like to see that catch on in the USA. 😀
I was surprised that the first citation was only in 1985 – somehow it seems (at least to me) to have been around for much longer.
But I have always regarded “numpty” as having a mild and somewhat affectionate connotation. It is a word I would use when one of my children has done something stupid, and probably not intended to cause offence or distress – “Oh, you numpty”. I certainly wouldn’t use it to describe
Donald (or Boris for that matter).
I’ve never liked the word, and would not use it. It has a certain unpleasant quality, and would normally only be used by one.
I caught part of this episode recently: [from Wikipedia] “800 Words is an Australian–New Zealand comedy-drama television ….. The other kids in class mock him as he has joined the ‘numpties’. This group include Lindsay who is repeating a year, and Billy (who is clever but in specific areas like bird migration)…”
As an aside, my favourite description for the relatively new Scottish Parliament is “the Numptorium”