Green’s Dictionary of Slang

weathercock n.

the head.

1821		‘An Amateur’ Real Life in London I 459–60: That vas a good one, it has altered her weather-cock and shifted her wind.
1858		A. Mayhew Paved with Gold 191: Before this round was completed, Jack took a heavy instalment on his ‘turret,’ which must have damaged his ‘weathercock.’.