Where does “mat-” come from?

mat- (Proto-Balto-Slavic) comes from Proto-Indo-European mē- — to measure; endeavour, will, temper; many.

Ancestry of “mat-”, step by step

Step	Language	Word	Meaning
1	Proto-Indo-European	mē-	to measure; endeavour, will, temper; many

mat- — Etymology, Origin & Word History | Etymology Explorer