Green’s Dictionary of Slang

front door n.

the female genitals.

[US]Life in Boston & N.Y. (Boston, MA) 28 June n.p.: The sensation produced by Holmes knocking at her front door, will be liable to reveal some little responsibility in [...] nine months.
[UK]Farmer & Henley Sl. and Its Analogues.
[US]Kokomo Arnold ‘Let Your Money Talk’ 🎵 If you go to the butcher to get your sausage grind, / If he can’t get it in the front door / He don’t want it behind.
[US]Maledicta IV:2 (Winter) 183: Examples here include [...] front door.
[US]‘Touré’ Portable Promised Land (ms.) 62: I’d never heard ’tain’t used to refer to female anatomy — not the front door or the back door but a mysteriously alluring, unclassifiable, scary region between a woman’s legs.

In compounds

front-door man (n.)

(US) in the context of adultery, the husband, the regular male partner.

[US]‘Randy Everhard’ Tattoo of a Naked Lady 207: Times like this, a bastard in my position can’t help but thinking about the front-door man. If I can kick his ass, I’ll do his woman.
front-door mat (n.)

the female pubic hair.

[US]Maledicta IV:2 (Winter) 183: Front doormat = pubic hair – ‘because they always say Welcome’.

In phrases