Green’s Dictionary of Slang

numpty n.

(UK Und./Scot.) a fool, an incompetent.

M. Munro The Patter—Another Blast.
D. Ross Xenophobe’s Guide to the Scots 16: One feeble utterance can mark a person down as a brainless ‘numpty’ for life.
[UK]C. Brookmyre Be My Enemy 114: It [i.e. a joke] was a sheer belter because it illustrated that [...] Rory could also be a blundering numpty.
[UK]N. ‘Razor’ Smith Raiders 246: Giving a gun to some numpty who might shoot his own foot off [...] is a no-no.
[UK]Sun. Times 19 Dec. 5/1: I had expected nothing less of the leader of the world’s numptiest radicals.
[UK]S. Kelman Pigeon English 100: Your sister’s a numptie, man. She thinks she’s a chicken.
[UK]Times 29 June [Internet] [Corbyn] gets his revenge by looking like an utter numpty? Funny kind of revenge.