Kristin Cavallari -- Jay Butt-ler ... Buns Out In Mexico (PHOTO)

Here's Jay Cutler's naked ass right in your face ... courtesy of his wife, Kristin Cavallari, who gave us a view of the QB's backfield while the two vacation in Mexico.

Jay is currently jobless, but it doesn't seem to be getting in the way of his off-season travels (or his squats, not bad dude).

Cavallari tagged Tony Scheffler -- a former NFL player -- in the pic, and we're not sure why.