1. [n]  the fleshy part of the human body that you sit on
2. [n]  person who does no work; "a lazy bum"
3. [n]  a person who is deemed to be despicable or contemptible; "only a rotter would do that"; "kill the rat"; "throw the bum out"; "you cowardly little pukes!"
4. [n]  a disreputable vagrant; "a homeless tramp"; "he tried to help the really down-and-out bums"
5. [adj]  (informal) of very poor quality
6. [v]  ask for and get free; be a parasite

1. \Bum\, n. [Contr. fr. bottom in this sense.]
   The buttock. [Low] --Shak.
   

2. \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.
   

3. \Bum\, n.
   A humming noise. --Halliwell.
   

1. To make highly efficient, either in time or space, often at the expense of clarity. "I managed to bum three more instructions out of that code." "I spent half the night bumming the interrupt code." In elder days, John McCarthy (inventor of Lisp) used to compare some efficiency-obsessed hackers among his students to "ski bums"; thus, optimisation became "program bumming", and eventually just "bumming".

2. To squeeze out excess; to remove something in order to improve whatever it was removed from (without changing function; this distinguishes the process from a featurectomy).

3. A small change to an algorithm, program, or hardware device to make it more efficient. "This hardware bum makes the jump instruction faster."

Usage: now uncommon, largely superseded by v. tune (and tweak, hack), though none of these exactly capture sense 2. All these uses are rare in Commonwealth hackish, because in the parent dialects of English "bum" is a rude synonym for "buttocks".

[Jargon File]