Green’s Dictionary of Slang

bonnet n.¹

1. the vagina.

1790		‘Bumper Allnight. Esquire’ Honest Fellow 44: Pray don’t take me wrong, / The theme of my song, / Isn’t that with flounces upon it' / [...] /I mean what all girls have, a bonnet, brave boys, / A flesh purse, by some call’d a bonnet.
c.1850		Peeping Tom (London) 18 70/2: He’s got his hand upon my bonnet.

2. a woman.

1812

Vaux Vocab. of the Flash Lang.

In compounds

bonnet skewer (n.)

a (large) hat pin.

1889		Kipling ‘The Hill of Illusion’ in Under the Deodars 90: What do you wear these huge dagger bonnet-skewers for? They're big enough to kill a man with.
1948		Southwest Times (Pulaski, VA) 15 June 14/1: The damsel of today carries neither parasol nor bonnet skewer, but that, hasn't cut down the occupational hazards of mashing.