Green’s Dictionary of Slang

1. any drink having a dense consistency, e.g. porter, cocoa, coffee.

1861		M.E. Braddon Trail of the Serpent 183: The regulation coffee [...] commonly known to the inmates of the asylum as ‘the thick.’.
1887		J.W. Horsley Jottings from Jail 26: Lads, your only friend here is your brown lofe and pint of thick.
c.1895		Albert Chevalier ‘The God and The Star’ in Before I Forget (2009) 231–2: I’ve just left King Charles an’ Oliver Cromwell ’avin’ a cup o’ thick an’ a doorstep at the corfee stall.
1906		O.C. Malvery Soul Market 152: Now then , miss, ’arf of thick, three doorsteps, and a two-eyed steak.
1932		F. Jennings Tramping with Tramps 147: Rasher and fingers, a slab and a pint of thick!
1937		E. Raymond Marsh 328: How could you do with a nice two-eyed steak, eh, and a couple ’a’ doorsteps, with a nice pint of thick to wash it all dahn with, eh?

2. (orig./mainly juv.) a fool, an ignoramus.

1857		T. Hughes Tom Brown’s School-Days (1896) 127: I told you how it would be. What a thick I was to come!
1866		Mansfield School-Life at Winchester College (1870) 238: Thick – Stupid [...] thus a Dunce.
1925		S. O’Casey Juno and the Paycock Act III: The thick made out the Will wrong.
1942		(con. 1890s) S. O’Casey Pictures in the Hallway 10: With me landin’ first in Wicklow, an’ dhriven out again be th’ ignorant thicks.
1956		B. Behan Quare Fellow (1960) III i: Sure it’d only be a thick wouldn’t improve his knowledge when an older man would be willing to tell him something that would be of benefit to him in his career.
1961		(con. 1930s) D. Behan Teems of Times and Happy Returns 120: Yer an awful thick, Giggles.
1970		C. Brown Down All the Days 132: ‘How do you know?’ challenged another boy [...] ‘Didn’t I see it, you thick?’ declared Charley.
1989		H. Leonard Out After Dark 58: What the hell are you doing in Fourth Year, you thick, you?
1991		R. Doyle Van (1998) 386: The dog was no thick. He could nearly talk, the noises he made somethimes when he wanted a biscuit or a chip.
1999		(con. 1916) R. Doyle A Star Called Henry (2000) 100: You thicks! You bloody eejits!

3. a thick slice of buttered bread.

1891		G.R. Sims ‘The People’s Palace’ Dagonet Ditties 134: They made it a coffee palace, with scones and a plate of ‘thick.’.
1911		G.R. Sims Off the Track in London 189: One of these, an eating-house, boldly announces itself as ‘Ye Olde Jimmy Thicks,’ and I take it that the ‘thicks’ are the slices of bread and butter.

4. (US black) one who has a muscular, well-developed physique.

1994

G. Smitherman Black Talk.