BM (also "B.M.") is an abbreviation
for bowel movement
, hence the pun
that a person in a cold area using an unheated toilet
will have an icy BM
In the Library of Congress Classification System, call numbers beginning with BM are assigned to items about Judaism.
I doubt this has any connection to the historical fondness of canonical Jewish mothers for prescribing an enema as a cure for various ailments, but it makes you wonder.