The Collaborative International Dictionary of English v.0.48:

BM \BM\ n.
   an abbreviation for bowel movement; -- used as an informal
   euphemism, especially in hospitals. [informal, abbrev.]
   [PJC]

WordNet (r) 3.0 (2006):

BM
    n 1: solid excretory product evacuated from the bowels [syn:
         fecal matter, faecal matter, feces, faeces, BM,
         stool, ordure, dejection]
    2: a euphemism for defecation; "he had a bowel movement" [syn:
       bowel movement, movement, bm]

The Free On-line Dictionary of Computing (19 January 2023):

bm

    The country code for Bermuda.

   (1999-01-27)