Well, at first I was going to go as Bob's Big Boy, a suggestion courtesy of my friend Kate's cool boyfriend. But somehow, that didn't quite work out. So I asked myself, "Self?" says I, "what is a comfortable option for a costume, preferably one where I can hump people's legs and wear a diaper?" And thus, this picture:
The key to a successful costume: a good prop.
(See also, dirty old man balls.)