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.