Green’s Dictionary of Slang

(US gay) a loose anus; thus Grand Canyon Suite, noisy, sloppy-sounding anal intercourse.

1972

B. Rodgers Queens’ Vernacular 99: Grand Canyon loose-fitting anus, as complained about by an active partner ‘Talk about the Grand Canyon: her ass is as wide as the natural wonder itself.’ Syn: Lincoln Tunnel. Grand Canyon Suite noisy, sloppy-sounding intercourse.