biffin, n.² — Green’s Dictionary of Slang

biffin n.²

also biffon

[fig. use of biff v. (1), i.e. one is biffing against it]

1. (also biffin bridge) the perineum, that area between the scrotum and anus or the vagina and anus.

1997		Roger’s Profanisaurus in Viz 87 Dec. n.p.: biffin n. Sweat secreted in the biffin bridge (qv) area during intercourse. [Ibid.] n.p.: biffon n. That part of the female anatomy between the bottom and the vagina, which the man’s balls biff on during intercourse.
2005		Roger’s Profanisaurus Rex.

2. sweat secreted in this area during intercourse.

1997		Roger’s Profanisaurus in Viz 87 Dec. n.p.: biffin n. Sweat secreted in the biffin bridge (qv) area during intercourse.
2005		Roger’s Profanisaurus Rex.