(Also known as a Flart) when you fart in the bathtub & a little bit of water gets in your ass. You then get out of the tub & fart in such a way it sounds like a mother-in-law spitty kiss.
"Phumpt, tub fart, mother-in-law's kiss
You know like a fart after a tub fart? It's a little moist kiss.