Bum (bŭm), n. [Contr. fr. bottom in this sense.] The buttock. [Low] Shak.
[1913 Webster]

bum (bŭm), v. t. To borrow without intention of returning; to cadge; as, to bum a cigarette; to bum a cup of coffee; -- usually with inexpensive items as the object. [informal]
[PJC]