Is it ensure, insure, or assure?
Old English innung "a taking in, a putting in," gerundive of innian "get within, put or bring in," from inn (adv.) "in" (see in). Meaning "a team's turn in a game" first recorded 1735, usually plural in cricket, singular in baseball.
[fr baseball; the British term, fr cricket, is always innings and is found by 1836]