Green’s Dictionary of Slang

canyon n.

1. the vagina (cf. yodel in the canyon (of love) v.

c.1935		implied in yodel in the canyon (of love) v. (2)
1970		in P.R. Runkel Law Unto Themselves 258: I finally got my nose warm in ’er canyon.
1980		E. Folb Runnin’ Down Some Lines 152: Who’s gonna flood yo’ hole, fill yo’ canyon up? Me!

2. the anus.

1995

(con. 1985–90) P. Bourjois In Search of Respect 38: They take your ass and stuff it with some mad concrete. They fill your canyon. Word!

In phrases

dive in the canyon (v.)

to perform cunnilingus.

c.2003

‘Euphemisms for Cunnilingus’ at DribbleGlass.com [Internet] Dive in the bushes / Dive a muff / Dive in the canyon.