Is it ensure, insure, or assure?
early 14c., "a split, act of splitting," from a Scandinavian source (cf. Danish and Norwegian rift "a cleft," Old Icelandic ript (pronounced "rift") "breach;" related to Old Norse ripa "to break a contract" (see riven). Figurative use from 1620s. Geological sense from 1921. As a verb, c.1300.