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".
Usage: now uncommon, largely superseded by v. tune (and tweak, hack), though none of these exactly capture sense 2.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".