Green’s Dictionary of Slang

1. the vagina, esp. when glimpsed beneath clothes.

1989

H. Rawson Dict. of Invective (1991) 39: Beaver probably originated as a street cry — a code word that men could call out to other men, advising them that if they looked sharp, a woman without underpants might be seen; cooze and zitz have been used the same way.

2. see zit n.² (1)