As nouns the difference between knave and bum
is that
knave is (archaic) a boy; especially, a boy servant 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 .