A term of utter contempt for a worthless, meretricious person, a no-account, with the implication that
s/he is of low moral character.
A "bum" has
long been an Americanism for a
tramp, vagabond or homeless person (and lacking in the "backside" meaning as in the U.K.). The "crumb" may come from the irritating or useless character of bread crumbs or
toaster crumbs, but it is possible (despite the spelling) that the first syllable derives from "crummy" as in worthless, detestable. The internal rhyme solidifies the expression.
.
"I have no use for my daughter's ex-
husband. Ever since the divorce he drifts from job to job and is always behind on
child-support payments. He doesn't even send his
child a birthday card! As far as I'm concerned, he's a complete crumb-bum."
.