"buttocks," late 14c., "probably onomatopœic, to be compared with other words of similar sound and with the general sense of 'protuberance, swelling.' " [OED]
"dissolute loafer, tramp," 1864, American English, from bummer "loafer, idle person" (1855), probably from German slang bummler "loafer," from bummeln "go slowly, waste time." Bum first appears in a German-American context, and bummer was popular in the slang of the North's army in the American Civil War (as many as 216,000 German immigrants in the ranks). Bum's rush "forcible ejection" first recorded 1910.
"of poor quality," 1859, American English, from bum (n.). Bum steer in figurative sense of "bad advice" attested from 1901.
[probably fr German Bummler, ''loafer'']
The buttocks or anus; ass •More common in British usage: after getting a shot of something in her bum
[late 1300s+; fr Middle English ''anus'']
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Usage: now uncommon, largely superseded by v. tune (and tweak, hack), though none of these exactly capture sense 2.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".