bum - 1864, Amer.Eng., from bummer
"loafer, idle person" (1855), possibly an
extension of the British word for "backside"
(similar development took place in Scotland,
16c.), but more prob. from Ger. slang
bummler "loafer," popular in the North's
army in the Civil War (many Ger. immigrants
in the ranks), from bummeln "go slowly,
waste time." Bum's rush "forcible ejection"
first recorded 1910. Bummer "bad
experience" is 1960s slang.

Incidentally, in German, a slow train that stopped at every station was called a "Bummelzug".