As nouns the difference between buttock and bum
is that buttock
is (usually|in the plural) each of the two large fleshy halves of the posterior part of the body between the base of the back, the perineum and the top of the legs while bum
is the buttocks or bum
can be (north america|colloquial) a hobo; a homeless person, usually a man or bum
can be (dated) a humming noise or bum
can be (obsolete) a bumbailiff.
As a verb bum is
(uk|transitive|colloquial) to sodomize; to engage in anal sex or bum
can be (colloquial) to ask someone to give one (something) for free; to beg for something or bum
can be to depress; to make unhappy or bum
can be to make a murmuring or humming sound.
As an interjection bum is
) an expression of annoyance.
As an adjective bum is
of poor quality or highly undesirable .