When a man has sex with a girl on her period, he takes his dick out and rubs it on her lips making it look like The Joker from The Dark Knight. When she gets pissed, ask her "why so serious?"
"Joe gave me The Joker last night, it was disgusting."
"HAHAHAHAHA!!!"
"Thats not funny!!"
"Why so serious?"
Taking a fat dump. Usually refers to a dump that is explosive, or runny. Noise and a deep senses of accomplishment and refreshment often accompany a Jane Fonda.
-"Dude! It smells in the bathroom!"
-"Oh, sorry man. I just did the Jane Fonda."
"Man, that IU student totally smells like he did the Jane Fonda in his pants!"