Green’s Dictionary of Slang

1. the mind, the brain.

1850		‘Ned Buntline’ G’hals of N.Y. 199: Yes, happy, ’cause they brought no bad memories to a feller’s thinky, (brain).
1890		Dead Bird (Sydney) 9 Aug. 5/3: Utilise your thinker in remembering that you owe me a pair of gloves.
1895		E. Townsend Chimmie Fadden Explains 19: Say, dat gives me a jolt in me tinker, see?
1903		A. Binstead Pitcher in Paradise 146: At last he utterly gets his thinker out of whack and goes back to the villa.
1904		‘Dads Wayback’ in Sun. Times (Sydney) 31 Jan. 3/5: ‘I’se often been stuck up in me thinker [...] wonderin’ wot we wears clothes fer’.
1910		O. Johnson Varmint 300: I may not be a tin sport but I keep my thinker going all the time.
1914		Bulletin (Sydney) 26 Nov. 44/1: I stays there a week while they fixes me thinker up, an’ then they fires me. Sez they: ‘We can’t keep a cove till the end of his little finger grows again; so ’ook it.’.
1949		R. Chandler Little Sister 110: What’s on the thinker, pal?
1999		J. Ridley Love Is a Racket 77: All those words got mixed up in my thinker.

2. (Irish) an intellectual challenge.

2021

P Howard Braywatch 137: No one says anything for ages. It’s a real thinker, that one.