A bum-drummer is a male person who partakes in the drumming of the bum, ie a batty boy, queer, homo, shit stabber. or in the conventional wording a homosexual.
Technically a homosexual who indulges in giving anal sex (also a heterosexual male or even female who enjoys giving anal sex). However common use of the word refers to someone, usually a friend or aqaintance, you wish to ridicule.