Green’s Dictionary of Slang

1. (UK teen) a hard worker.

1888		Times 1 Feb. 12/2: Remember the many epithets applied to those who, not content with doing their work, commit the heinous offence of being absorbed in it. Every school, every college has had its choice nickname for this unfortunate class... such as a ‘sap’, a ‘smug’, a ‘swot’, a ‘bloke’, a ‘mugster’ [F&H].
1900		Boy’s Own Paper 17 Nov. 110: It is seldom that the man who plays no game does any successful work. The ‘smug’ as he is called, is not often high on the list.
1912		P.C. Wren Dew & Mildew 57: Ignored by the sporting set as a hopeless smug.

2. an intellectual.

1890		‘’Arry on the Sincerest Form of Flattery’ in Punch 20 Sept. 144/2: When smugs makes me out a ‘philolergist,’ — snuffers! it do make me grin!
1910		C.L. Cullen Eddy 13: Are you afraid she has become a bluestocking? Or maybe a frump? Or, worse still, what you call one of the anointed smugs?