Is it ensure, insure, or assure?
late 14c., "injurious, dangerous," from Old French inconvénient (13c.), from Latin inconvenientem (nominative inconveniens) "unsuitable, not accordant, dissimilar," from in- "not, opposite of" (see in- (1)) + convenientem (see convenient). In early 15c., "inappropriate, unbecoming, unnatural;" also, of an accused person, "unlikely as a culprit, innocent." Sense of "troublesome, awkward" first recorded 1650s.