Is it ensure, insure, or assure?
1680s (v.) "to assume a formal, precise demeanor," perhaps from French prim "thin, small, delicate," from Old French prim "fine, delicate," from Latin primus "finest," literally "first" (see prime (adj.)). Later, "deck out, dress to effect" (1721). Attested as a noun from 1700. The adjective, the sole surviving sense, is from 1709. A cant word at first. Related: Primly; primness.