Green’s Dictionary of Slang

stupid, dull.

2000		Guardian G2 6 Jan. 5: Surely he cannot be saying, in what I’m afraid is known as a ‘mongy’ voice: ‘Urrrgh, my name’s David Beckham’.
2000		Indep. on Sun. Reality 19 Mar. 23: Liam Gallagher is a mong boy.
2005		B. Hare Urban Grimshaw 50: You mongy-faced bastard.