Webster’s Revised Unabridged Dictionary · 1913
Bum
Bum, n. [Contr. fr. bottom in this sense.] The buttock. [Low] Shak.
Bum, v. i. [imp. & p. p. Bummed; p. pr. & vb.n. Bumming.] [See Boom, v. i., to roar.] To make murmuring or humming sound. Jamieson.
Bum, n. A humming noise. Halliwell.