Traveling without a fixed destination or plan, living without much money; or asking someone for something you need.
From 'bum' (1864 Americanism, possibly from German 'Bummler' meaning a lazy person). The sense of 'asking for' developed from vagrants bumming cigarettes and money.
Bumming a cigarette became such a common phrase that it eclipsed the deeper meaning—vagrants who literally bummed (wandered) across America in the 1920s-30s documented entire social realities in their slang.
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