bum: There are two distinct words bum in English. By far the older, ‘buttocks’, is first recorded in John de Trevisa’s translation of Ranulph Higden’s Polychronicon 1387: ‘It seemeth that his bum is out that hath that evil [piles]’. It is not clear where it comes from. The other, ‘tramp, loafer’, and its associated verb ‘spend time aimlessly’ [19], chiefly American, probably come from an earlier bummer, derived from the German verb bummeln ‘loaf around, saunter’ (familiar to English speakers from the title of Jerome K Jerome’s novel Three Men on the Bummel 1900, about a jaunt around Germany).
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 "loafer, idle person" (1855), probably from German slang bummler "loafer," from bummeln "go slowly, waste time." Bum first appears in a German-American context, and bummer was popular in the slang of the North's army in the American Civil War (as many as 216,000 German immigrants in the ranks). Bum's rush "forcible ejection" first recorded 1910.
bum (adj.)
"of poor quality," 1859, American English, from bum (n.). Bum steer in figurative sense of "bad advice" attested from 1901.
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.). Meaning "to feel depressed" is from 1973, perhaps from bummer in the "bad experience" sense. Related: Bummed; bumming.