Is it ensure, insure, or assure?
Old English spir "sprout, shoot, stalk of grass," from Proto-Germanic *spiraz (cf. Old Norse spira "a stalk, slender tree," Middle Low German spir "a small point or top"), from PIE *spei- "sharp point" (see spike (n.1)). Meaning "tapering top of a tower or steeple" first recorded 1590s (a sense attested in Middle Low German since late 14c. and also found in the Scandinavian cognates). The verb is first recorded early 14c.