French word merrain comes from Latin materia, Latin -men (Forms neuter nouns of the third declension.)

Detailed word origin of merrain

Dictionary entry	Language	Definition
materia	Latin (lat)	(figurative) source, origin. Matter; material; component stuff; substance. Timber.
-men	Latin (lat)	Forms neuter nouns of the third declension.
materiāmen	Vulgar Latin (la-vul)
merrain	French (fra)	Oak stave (used to make barrels etc).

Words with the same origin as merrain