Green’s Dictionary of Slang

mug adj.

[mug n.1 (2a)]

foolish, stupid.

[UK] ‘’Arry’s Visit to the Moon’ in Punch Christmas Number in P. Marks (2006) 168: A whole howling mix-up of ‘mug’ booky, dog-owner and rough.
[UK]J.D. Brayshaw Slum Silhouettes 34: Oh well, if he’s mug enough to take up wiv her, let him look after her.
[Aus]Sydney Sportsman (Surry Hills, NSW) 27 Mar. 1/2: So the mug pony patron parted up 7s 6d for Mountain’s entrance ticket.
[UK]‘Doss Chiderdoss’ ‘An Improved Understanding’ Sporting Times 14 Jan. 1/4: ’Twas a match in the making, folks hadn’t a doubt; / And her people seemed willing to bring it about, / Though the suitor was of the ‘mug’ brand.
[Aus]Bulletin (Sydney) 15 Dec. 35/1: Why, if yer silly mug enuff to go diggin’ big ’oles ’arf over Melbourne with a pick an’ shovel, you deserve orl yer likely to git [...].
[UK]‘Sapper’ Human Touch 83: If some one was mug enough to try and spot the lady coming up from Epsom in the train, when Pete was manipulating the cards – well, surely enough has been said about the three-card trick.
[UK]F. Anthony ‘Helping Out Gus’ in Me And Gus (1977) 11: I’m always mug enough to accept the offer.
[Aus]S.L. Elliott Rusty Bugles I iii: You’re a mug liar.
[NZ]G. Slatter Gun in My Hand 213: Mug ape. What’s eating ya? Clueless bastard!
[Aus]A. Chipper Aussie Swearers Guide 62: Note the national fondness for the word mug, as in mug copper, mug gunner and ‘You’ve gotta protect the mugs’.
[Ire]J. Morrow Confessions of Proinsias O’Toole 5: Poor deluded mug me!
[Aus]J. Byrell (con. 1959) Up the Cross 103: He’d never been seen to [...] make any mug moves.
[Aus]J. Byrell Lairs, Urgers & Coat-Tuggers 119: The official excuse he offered for that sudden mug move was that the fly-poop 49 kg [...] was ‘too much of an impost for this horse’.
[UK]N. ‘Razor’ Smith Raiders 84: He was in for what I class as futile, pointlessly viiolent mug-crimes.