What does it mean?

Does anyone know its origins?

Is it common?

It's common and slang. I am so young that I can only trace it from my youth. To be bummed = to be bummed out (i.e. suffer a bumming ( bum = disappointing ) experience. Here's the earlier etymology from Online:

bum (2) "dissolute loafer, tramp," 1864, Amer.Eng., from bummer "loafer, idle person" (1855), possibly an extension of the British word for "backside" (similar development took place in Scotland, 1540), but more prob. from Ger. slang bummler "loafer," from bummeln "go slowly, waste time." Bum first appears in a Ger.-Amer. context, and bummer was popular in the slang of the North's army in Amer. Civil War (as many as 216,000 Ger. immigrants in the ranks). Bum's rush "forcible ejection" first recorded 1910. Bummer "bad experience" is 1960s slang.