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.