Is it ensure, insure, or assure?
1580s (earlier in various Latin and French phrases), from Latin per "through, during, by means of, on account of, as in," from PIE root *per- (1) "Base of prepositions and preverbs with the basic meanings of 'forward,' 'through,' and a wide range of extended senses such as 'in front of,' 'before,' 'early,' 'first,' 'chief,' 'toward,' 'against,' 'near,' 'at,' 'around'" [Watkins]. Cf. Sanskrit pari- "around, about, through," pura "before, formerly;" Avestan pairi- "around," paro "before;" Old Persian pariy; Hittite para- "on, forth;" Greek peri "around, about, near, beyond," paros "before," para "from beside, beyond," pro "before;" Latin pro "before, for, on behalf of, instead of," porro "forward," prae "before;" Old English fore (prep.) "before, in front of;" (adv.) "before, previously;" German vor "for;" Old Church Slavonic pra-dedu "great-grandfather;" Russian pere- "through;" Lithuanian per "through;" Old Irish air- Gothic fair-, German ver-, Old English fer-, intensive prefixes.
Thoroughly; completely; intensely: perfuse.
Containing an element in its highest oxidation state: perchloric acid.
Containing a large or the largest possible proportion of an element: peroxide.
Containing the peroxy group: peracid.