"loafer, idle person," 1855, possibly an extension of the British word for "backside" (similar development took place in Scotland by 1540), but more probably from German slang bummler "loafer," agent noun from bummeln "go slowly, waste time."
According to Kluge, the German word is from 17c., and the earliest sense of it is "oscillate back and forth;" possibly connected to words in German for "dangle" (baumeln), via "back-and-forth motion" of a bell clapper, transferred to "going back and forth," hence "doing nothing." Meaning "bad experience" is 1968 slang.
"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 "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.
"of poor quality," 1859, American English, from bum (n.). Bum steer in figurative sense of "bad advice" attested from 1901.
An exclamation of dismay: Ms. Riner is too docile, too scared, too unsexy for the role, and— bummer!—there seems to be a real possibility that she's innocent
modifier: Zonk is rushed to the Woodstock bummer tent
noun[first noun sense probably fr German Bummler, ''loafer'']
beach bum, crumb-bun, skid row bum, stewbum, stumblebum
[probably fr German Bummler, ''loafer'']
The buttocks or anus; ass •More common in British usage: after getting a shot of something in her bum
[late 1300s+; fr Middle English ''anus'']