Bum
mid 19th century: probably from bummer.
wiktionary
Attested since the 1300s, [1] [2] [3] as Middle English bom [1] (found in John Trevisa's 1387 Translation of the 'Polychronicon' of Ranulph Higden, "his bom is oute"), of uncertain origin. [1] Sometimes suggested to a shortening of botme, botom, bottum(“bottom”), but this is contradicted by the fact that bottom is not attested in reference to the buttocks until the late 1700s. [4] [5] Suggested by some old [4] and modern references to be onomatopoeic. [3] Compare also Old Irish, Scottish Gaelic bun(“base, bottom”).
1864, back-formation from bummer, from German Bummler(“loafer”), from bummeln(“to loaf”).
Back-formation from bum out.
See boom.
Abbreviations.
etymonline
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.
