Middle English do, first person singular of Old English don "make, act, perform, cause; to put, to place," from West Germanic *don (cognates: Old Saxon duan, Old Frisian dua, Dutch doen, Old High German tuon, German tun), from PIE root *dhe- "to put, place, do, make" (see factitious).Use as an auxiliary began in Middle English. Periphrastic form in negative sentences ("They did not think") replaced the Old English negative particles ("Hie ne wendon"). Slang meaning "to do the sex act with or to" is from 1913. Expression do or die is attested from 1620s. Compare does, did, done.

first (and last) note of the diatonic scale, by 1754, from do, used as a substitution for ut (see gamut) for sonority's sake, first in Italy and Germany. U.S. slang do-re-mi "money" is from 1920s, probably a pun on dough in its slang sense of "cash."