Green’s Dictionary of Slang

1. an act of defecation; also as v.

1962		(con. WWII) J.O. Killens And Then We Heard The Thunder (1964) 12: He had had seven or eight B.M.s, with the latrine orderly standing there. [Ibid.] 312: All through this time when he wasn’t B.M.ing or scared to death, he was asking himself, Why am I here?
1967–8		Baker et al. CUSS 73: B.M. have a / Bowel movement / defecate.
1978		S. King Stand (1990) 595: The fine BM I had last night, You was right, those dates turned the trick.
1993		S. King Dolores Claiborne 32: I knew she was holdin onto one gut-buster of a b.m.
1997		Mad mag. July 13: IBM. We all BM.

2. a piece of excrement.

1967		I. Rosenthal Sheeper 195: [of lizards] One B.M. per week and odorless. The ideal pet.
1968		G. Cuomo Among Thieves 119: Jimmy made B.M. in the potty.
1997–2001		Online Sl. Dict. 🌐 B.M. n 1. feces. Origin: abbreviation of ‘bowel movement’. (‘I just laid a nasty B.M!’).
2005		Mad mag. Dec. 36: Let’s see Dr Finklefart Goldblob tippy-toe through a box of crapballs every time she has to have a B.M.