Is it ensure, insure, or assure?
Old English sparian "to refrain from harming, to allow to go free," from the source of Old English spær "sparing, frugal," from Proto-Germanic *sparaz (cf. Old Frisian sparia, Old Norse spara, Old High German sparon "to spare"). Meaning "to dispense from one's own stock" is recorded from early 13c. Related: Spared; sparing.
"kept in reserve, not used," late 14c., from spare (v.). Old English had spær "spare, frugal." In reference to time, from mid-15c.; sense of "flimsy, thin" is recorded from 1540s. Spare part is attested from 1888.
"extra thing or part," 1640s, from spare (v.). Middle English noun sense was "mercy, leniency" (early 14c.). Bowling sense of "a knocking down of all pins in two bowls" is attested from 1849, American English.