Is it ensure, insure, or assure?
c.1300, "freed from dregs or lees" (of ale, wine, etc.), i.e. "having stood long enough to clear," cognate with Middle Dutch stel "stale" (of beer), and probably ultimately from Proto-Germanic base *sta- "stand," the source of Old English standan "to stand," Perhaps via Old French estaler "halt," from Frankish *stal- "position" (see stall (n.1)). The meaning "not fresh" is first recorded late 15c. Figurative sense (of immaterial things) is recorded from 1560s. Related: Staleness.