"buttocks," late 14c., "probably onomatopœic, to be compared with other words of similar sound and with the general sense of 'protuberance, swelling.' " [OED]
"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.
"of poor quality," 1859, American English, from bum (n.2). Bum steer in figurative sense of "bad advice" attested from 1901.