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.

A term used to reference cannabis/marijuana/weed.

Someone who is acting like a complete arse, affectionate insult

Someone who isn't doing the work they're supposed to, who's being lazy.

Not just being a bum but the lifestyle. Going around care-free, looking any type of way, acting any type of way.

when you eat in bed, then later have sex in it, you will get the crumb bum.

a person with a particularily negative attitude; often seen in irish men who believe in karma.