When your ass crack gets all slimy, sweaty and stinky because you didn't wipe good enough.
I took a shit this morning and went to the gym. Now I need to wipe because I have Mexican Ass.
A Prehistoric Donkey (4-legged hoofed animal) from the southwest or south of the border. Fossilized Remains of it were discovered by archaeologists at or near the La Brea Tar Pits.
On wikipedia. Type in the following key words: "La Brea Tar Pits" Scroll down Until you see a list of all of the prehistoric animals including the "Mexican Ass," that archaeologists found the remains of.