prim (adj.)

"formal, stiffly precise in speech or manners," 1709, the sole surviving sense of a word attested first as a verb (1680s) "to assume a formal, precise demeanor," a cant word of uncertain origin, perhaps from French prim "thin, small, delicate" (Old French prim "fine, delicate"), from Latin primus "finest," literally "first" (see prime (adj.)).

Later, "deck out with great nicety, dress to effect, form or dispose with affected preciseness" (1721). It also is attested as a noun from 1700, "formal, precise, or stuck-up person." Related: Primly; primness.