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
(
label) an expression of annoyance.
As an adjective bum is
of poor quality or highly undesirable .