Green’s Dictionary of Slang

1. as mount pleasant of Rome, the buttocks of a male homosexual.

1748–49

Cleland Memoirs of a Woman of Pleasure (1985) 158: Those globular, fleshy eminences that compose the mount-pleasants of Rome [...] stood display’d and expos’d to his attack.

2. the female genital area.

1748–49		Cleland Memoirs of a Woman of Pleasure (1985) 11: The soft silky down that had but a few months before put forth, and garnish’d the mount-pleasant of these parts.
1781		Belle’s Stratagem 17: My Letty would never sacrifice her pretty little Mount Pleasant to any of those news-paper rattlesnakes, if it was n‘t to soften their venemous pens.
1890–1904		Farmer & Henley Sl. and Its Analogues.
1980		Maledicta IV:2 (Winter) 185: Common examples [of sexual ‘addresses’] include Cockshire, Cock Inn, Cupid’s Alley, Hairyfordshire, Crown and Feathers, Shooter’s Hill, Mount Pleasant, Love Lane, etc.