a true stinky sanchez is just this. please excuse any racial undertones, but this is how i learned it. when a mexican or latino male can't grow the thin whispy mustache that his amigos can so he insers his finger in his butthole till he gets a chocolate chip on the end of said finger and then proceeds to paint a mustache on his upper lip.

The performer takes a finger and inserts it into their buttcrack until a healthy brown smudge is harvested. An unsuspecting victim is then carefully and quietly approached, and at the right moment the dirty digit is rubbed/smeared on the victim, in the thin space above their upper lip and below their nostrils.
The result, a brown and smelly temporary moustache, is known as the "stinky sanchez." Alternative titles include the "dirty hitler," "brown rollie" and "tom selleck."
For extra points, perform it on a sleeping victim.

When you take your finger and stick it between your crack and wipe it under someones nose.

to rub a selected finger all over your ass region the apply "stinky sanchez" on a un suspected persons beer or beverage. also known ass a dirty sanchez