1. the sexual act of following anal sex immediately with oral sex, which causes residual fecal matter to coagulate on the recipient's upper lip, resembling a Mexican handlebar moustache
2. Unexplained sexual position taught by South Park
character Mr. Garrison
to his kindergarten class.
(1) 'Naw, I gave that slut Jessica a Filthy Sanchez, dude, not the other way around!'
(2) 'Ike, do you know what a "Filthy Sanchez" is?'