Green’s Dictionary of Slang

an extremely hairy vagina.

1982		N. Keesing Lily on the Dustbin 40: Few girls desire ‘a forest’ or ‘mohair stockings’ and will assiduously ‘mow the lawn’ to control them.
1997		Roger’s Profanisaurus in Viz 87 Dec. n.p.: mohair knickers euph. A large minnie moo.