écrou

The French word "écrou" primarily means a nut in mechanical terms — the small, typically metal piece with a threaded hole that screws onto a bolt (vis) to fasten objects together.

Meanings:

1. Mechanical meaning:
   • An écrou is a nut, used with a bolt or screw to hold things tightly.
2. Figurative/colloquial meaning (less common):
   • Informally, "mettre quelqu’un à l’écrou" means to put someone in jail or prison (equivalent to "lock up" or "jail").

Synonyms and distinctions:

• Boulon: Means bolt or screw, the metal rod that fits with the nut (écrou).
• Vis: Means screw, a fastener that can work with or without a nut.
• Contre-écrou: A lock nut or jam nut, used to lock a preceding nut in place.

Summary:

• Use écrou to mean the machinery part — the nut.
• In slang/legal context, it may refer to confinement or imprisonment.
• It’s distinct from boulon (bolt) and vis (screw), which are related fastening pieces.

If talking about the jewelry part or fastening element, “écrou” is always the nut.