bum :

vt. 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, optimization became "program bumming", and eventually just "bumming".
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).
n. 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 Usage: now uncommon, largely superseded by v. tune (and n. tweak, hack), though none of these exactly capture sense
All these uses are rare in Commonwealth hackish, because in the parent dialects of English `bum' is a rude synonym for `buttocks'