When someone of a rather large exterior (aka fat person) strips down (naked) and goes swimming (once again, naked). A variation of Skinny-Dipping, as when the same occurance takes place, but with skinny people (**cough**anorexia**cough**).
When a skinny man has vaginal intercourse with an obese woman. The skinny man is on top while the obese woman is on the bottom therefore he is dunking his penis into her.