Is it ensure, insure, or assure?
word-forming element making verbs (e.g. darken, weaken) from adjectives or from nouns, from Old English -nian, from Proto-Germanic *-inojan (cf. Old Norse -na), from PIE adjectival suffix *-no-. Most active in Middle English.
assimilated to -p-, -b-, -m-, -l-, -r-, word-forming element meaning "in, into," from French and Old French en-, from Latin in- "in, into" (see in- (2)).
Also used with native elements to form verbs from nouns and adjectives, "put in or on" (encircle), also "cause to be, make" (endear), and used as an intensive (enclose). Spelling variants in French that were brought over into Middle English account for parallels such as assure/ensure/insure.
en- 2 or em-
In; into; within: enzootic.