Green’s Dictionary of Slang

the vagina.

1842		Exquisite 62 16/2: ‘Madam,’ said a husband to a lady in whose garden he was walking, ‘I should much like to pay a visit to your gro(o)ve of sweet retirement’ .
1889		C. Deveureux Venus in India I 95: I passed my middle finger in the groove, just tipping the awakened and slippery little clitoris, until I reached the entrance to the rich depths I sought for.
c.1900		Confessions of Lady Beatrice 22: The thongs would flick within my groove.
c.1935		‘Moon Mullins in “Nerts”’ [comic strip] in B. Adelman Tijuana Bibles (1997) 31: I ain’t the first one ever dropped it in your groove.
c.1940		‘Benny Goodman in “Swing on This!”’ [comic strip] in B. Adelman Tijuana Bibles (1997) 105: Keep it up Benny! You’re in the groove!