AM BM
Morning (AM) bowel movement (BM). The first poop of the day, almost always taken immediately upon awakening in the morning.

A "beer dump" is a type of AM BM, but not all AM BMs are beer dumps.
"I can't even start my day with some coffee until I've had my AM BM."

"I didn't have time for my AM BM this morning so I've had to hold that sh*t in all f*cking morning."