Is it ensure, insure, or assure?
1520s, back-formation from importunity, or else from Middle French importuner, from Medieval Latin importunari "to make oneself troublesome," from Latin importunus "unfit, troublesome," originally "having no harbor" (i.e. "difficult to access"), from assimilated form of in- "not, opposite of" (see in- (1)) + portus "harbor" (see port (n.1)). Related: Importuned; importuning. As an adjective from early 15c.