Bum, meaning buttocks, which is mainly an informal British usage has many compound variations from the word e.g. bum-boy, bum-beating, bum-sucker etc.
Etymology: Origin uncertain.
Probably onomatopoeic, to be compared with other words of similar sound and with the general sense of ‘protuberance, swelling’, e.g. bump n.1, bumb n. a pimple, modern Icelandic bumba belly of a cask or other vessel, French bombe bomb n. Compare also bum v.1 (The guess that bum is ‘a mere contraction of bottom’, besides its phonetic difficulties, is at variance with the historical fact that ‘bottom’ in this sense is found only from the 18th cent.)
Bum, meaning, a lazy person, a tramp, a loafer might be short for bummer:
Etymology: compare German bummler in same sense.
An idler, lounger, loafer. See also quots.
1855 Oregonian (Portland, Oregon) 27 Jan. 1/4 Come, clear out, you trunken loafer! Ve don’t vant no bummers here!
1856 San Francisco Call 25 Dec. ‘Pon my word I’m no bummer. I never ate a lunch in all my life without taking a square drink.
Etymology: < bum adj. + -er suffix1.
orig. and chiefly U.S.
An unpleasant or depressing experience, esp. one induced by a hallucinatory drug (= down trip n. at down adj. 1e); a disappointment or failure. Freq. as complement to subj.: to be a bummer.
1967 J. Didion in Sat. Evening Post 23 Sept. 26/3 I ask if he found a ride to New York. ‘I hear New York’s a bummer,’ he says.
1968 Electronic Music Rev. Jan. 19 The declaimed lyric is an example of what heads would call a ‘bummer’.
1968 T. Leary Politics of Ecstasy viii. 166 The Western world has been on a bad trip, a 400-year bummer.
Note that some of these OED entries are not too recent.