Green’s Dictionary of Slang

thinker n.

also thinky

the mind, the brain.

[US]‘Ned Buntline’ G’hals of N.Y. 199: Yes, happy, ’cause they brought no bad memories to a feller’s thinky, (brain).
[Aus]Dead Bird (Sydney) 9 Aug. 5/3: Utilise your thinker in remembering that you owe me a pair of gloves.
[US]E. Townsend Chimmie Fadden Explains 19: Say, dat gives me a jolt in me tinker, see?
[UK]A. Binstead Pitcher in Paradise 146: At last he utterly gets his thinker out of whack and goes back to the villa.
[US]O. Johnson Varmint 300: I may not be a tin sport but I keep my thinker going all the time.
[Aus]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.’.
[US]R. Chandler Little Sister 110: What’s on the thinker, pal?
[US]J. Ridley Love Is a Racket 77: All those words got mixed up in my thinker.

In phrases

blow one’s thinker (v.)

to lose one’s mind.

[US]Anaconda Standard (MO) 23 Sept. 5/3: Dat’s what bryan says, but dey insist dat he’s blowed his thinker.