bum

Concise Oxford English Dictionary © 2008 Oxford University Press:
bum1

▶noun Brit. informal a person's buttocks or anus.

– origin ME: of unknown origin.

Concise Oxford English Dictionary © 2008 Oxford University Press:
bum2 informal

▶noun N. Amer.

• 1 a vagrant.

• 2 a lazy or worthless person.

• 3 a devotee of a particular activity: a ski bum.

▶verb (bums, bumming, bummed)

• 1 get by asking or begging.

• 2 (bum around) chiefly N. Amer. pass one's time idly.

• 3 (usu. be bummed out) N. Amer. make (someone) feel upset or disappointed.

▶adjective bad; wrong: not one bum note was played.

– phrases
give someone the bum's rush chiefly N. Amer. forcibly eject someone.

– origin C19: prob. from bummer.