écrou

nut (that fits on a bolt), female screw

Masculinized form from Middle French escroue, from Old French escroe, from Latin scrōfa, originally “sow (female pig)”; compare Occitan escrofa (“screw nut”), Sicilian scrufina (“screw nut”). The change in meaning is also found in Spanish puerca, Portuguese porca, both “sow; screw nut”, and is based on the fact that a boar's penis has a screw-like tip, making the sow's vulva equivalent to a screw nut by analogy.

prison register, prison admission form; incarceration

“levée d'écrou”

Related: écrouer

From Middle French escroue (“register, file; piece, strip”) from Old French escroe, from Frankish *skrōda (“piece”). Compare English escrow, scroll.