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.
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: