bum (n.1)

"buttocks," late 14c., "probably onomatopœic, to be compared with other words of similar sound and with the general sense of 'protuberance, swelling.' " [OED]

bum (n.2)

"dissolute loafer, tramp," 1864, American English, from bummer (q.v.) "loafer, idle person" (1855), which is probably from German. Bum first appears in a German-American context, and bummer was popular during the American Civil War in the slang of the North's army (which had as many as 216,000 German immigrants in the ranks). There may also be influence or merging with bum (n.1) "buttocks," which was applied insultingly to persons from 1530s and is in Jamieson's 1825 Scottish dictionary. Bum's rush "forcible ejection" first recorded 1910.

bum (v.)

1863, "to loaf and beg," American English, a word from the Civil War, perhaps a back-formation from bummer "loafer," or from bum (n.2). Meaning "to feel depressed" is from 1973, perhaps from bummer in the "bad experience" sense. Related: Bummed; bumming.

bum (adj.)

"of poor quality," 1859, American English, from bum (n.2). Bum steer in figurative sense of "bad advice" attested from 1901.

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Definitions of bum from WordNet
bum (n.)
a person who is deemed to be despicable or contemptible;
throw the bum out
Synonyms: rotter / dirty dog / rat / skunk / stinker / stinkpot / puke / crumb / lowlife / scum bag / so-and-so / git
bum (n.)
a vagrant;
he tried to help the really down-and-out bums
Synonyms: tramp / hobo
bum (n.)
person who does no work;
a lazy bum
Synonyms: idler / loafer / do-nothing / layabout
bum (n.)
the fleshy part of the human body that you sit on;
Synonyms: buttocks / nates / arse / butt / backside / buns / can / fundament / hindquarters / hind end / keister / posterior / prat / rear / rear end / rump / stern / seat / tail / tail end / tooshie / tush / bottom / behind / derriere / fanny / ass
bum (v.)
ask for and get free; be a parasite;
Synonyms: mooch / cadge / grub / sponge
bum (v.)
be lazy or idle;
Her son is just bumming around all day
Synonyms: bum around / bum about / arse around / arse about / fuck off / loaf / frig around / waste one's time / lounge around / loll / loll around / lounge about
bum (adj.)
of very poor quality; flimsy;
Synonyms: cheap / cheesy / chintzy / crummy / punk / sleazy / tinny