Is it ensure, insure, or assure?
word-forming element meaning "before," from Old French pre- and Medieval Latin pre-, both from Latin prae (adverb and preposition) "before in time or place," from PIE *peri- (cf. Oscan prai, Umbrian pre, Sanskrit pare "thereupon," Greek parai "at," Gaulish are- "at, before," Lithuanian pre "at," Old Church Slavonic pri "at," Gothic faura, Old English fore "before"), extended form of root *per- (1) "beyond" (see per).
The Latin word was active in forming verbs. Also cf. prae-. Sometimes in Middle English muddled with words in pro- or per-.
Earlier; before; prior to: prenatal.
Anterior; in front of: preaxial.