From - "The Barnhart Concise Dictionary of Etymology
The Origins of American English Words."
Page 92 - "Bum - n. 1864, in American English, possibly
identical with earlier Scottish bum, (1540) lazy, dirty
person ( a special use of bum rump, before 1387, perhaps
borrowed from Middle Dutch bonne, modern Dutch bom bung)
and fussing with a shortened form of earlier English, bummer
loafer, idle person (1855) apparently alteration of German
bummler, from bummeln, to loaf showing influence of German
immigrants at the time.
v. 1863, American English, perhaps back formation from
bummer loafer, or from the noun (reinforced by bumming n.
adj. of poor quality, 1859, American English, from the noun;
also in bum steer bad advice (1920's), and in bummer bad
experience or situation (1969)"
"The Civil War Word Book," by Darryl Lyman states:
page 30 -
"Bum - The modern senses of bum originated just before
and during the Civil War: the adjective meaning worthless;
the verb meaning to loaf, beg, or wander; and the noun
meaning tramp, loafer, or sponger. The Civil War caused
an explosion in the use of the word; the war uprooted many
men an got them use to a wandering camp life."
More from Lyman -
"Bummer - (1) A loafer or sponger. From German Bummler,
(loafer). This sense of the word originated just before the
war, perhaps in the Far West. (2) During the war, an independent
forager, especially a soldier who left his ranks and plundered,
often as part of a raiding force."
"Bummers cap - The Union army's regulation fatigue or forage
cap. The name reflected the popular association of the cap
with Sherman's bummers."
European use of "bum", or a form of it, seems to stretch
back into the 1300's. The verb and adjective use of the
word in America seems to appear just prior to the Civil War.
IMO, it would seem natural to develop a descriptive word
into the noun for someone "of poor quality" and a "loafer"
or "idle person." And that is what seems to have been done.
Regards, Dave Gorski