Bum

• In ordinary (British) English, bum is a slightly vulgar expression meaning [the human] bottom, or buttocks. In the US this is more commonly callad a butt.
• In ordinary American English, bum has been a slang term for 'a vagrant', 'a tramp', 'a hobo' since the middle of the nineteenth century.
  • Before that, it was largely college slang for 'a drinking spree', 'a [usually alcoholic] entertainment'
• The current meaning, of 'atramp', has given rise to many derogatory meanings:
  • 'a worthless or contemptible person' (OED, 2018)
  • used epithetically, 'second rate'; in sporting contests 'destined to lose'; 'no-good', 'worthless', 'good-for-nothing'
  • As a verb, 'to bum' is 'to beg', 'to cadge'