Is it ensure, insure, or assure?
1590s, from Middle French frugal, from Latin frugalis, from undeclined adj. frugi "useful, proper, worthy, honest; temperate, economical," originally dative of frux (plural fruges) "fruit, produce," figuratively "value, result, success," related to fructus (see fruit). Sense evolved in Latin from "useful" to "profitable" to "economical." Related: Frugally.