Its when you fuck your woman so hard the night before, she wants to bake you pancakes in the morning!
Seriously, the term was derived from the term "pancakin' wednesdays," where, instead of the word "fuck," you use the forms of "pancake" all day.
Pancake (v.) - to fuck, bone, shag, screw, or anything of that nature, to a woman(or man) in an extreme, relentless fashion.
"I pancaked the shit outta that chick!"
"I don't make love... I PANCAKE!"
"I would pancake the shit outta her if i had the chance."
"Bro you wouldn't believe it! You remember Sherrie? I was pancaking all on that ass last night!"
BITCH, MAKE ME SOME FUCKING PANCAKES