Where does “mondain” come from?

mondain (Old French) comes from Late Latin mundanus, from Latin mundus, from Proto-Indo-European mewH- — to wash, wet; to make wet, wash; wet.

Ancestry of “mondain”, step by step

Step	Language	Word	Meaning
1	Late Latin	mundanus	worldly, mundane; cosmopolitan
2	Latin	mundus	clean, pure; neat; nice, fine, elegant,...
3	Proto-Indo-European	mewH-	to wash, wet; to make wet, wash; wet