'bum' (sense: vagrant/loafer) likely originates from German, specifically the word 'Bummler' meaning 'loafer' or 'wanderer', adopted into English slang in the 19th century.

'Bummler' (German) influenced American slang 'bum' (late 19th century) meaning 'vagrant' or 'idler'; the verb sense 'to bum' (to beg or mooch) developed from the noun.

Initially used for a 'loafer' or 'wanderer', the word broadened to refer to a 'vagrant', then to the verb meaning 'to beg or obtain (something) without paying'; a separate sense 'buttocks' arose independently in British English and became common in informal speech.