Where does “mastь” come from?

mastь (Proto-Slavic) comes from Proto-Slavic -tь, from Proto-Indo-European -tis — Derives abstract/action nouns from verb roots.

mastь (Proto-Slavic): ointment

Definitions

Step	Language	Word	Meaning
1	Proto-Slavic	-tь	Deverbative, creates nouns denoting act, state,...
2	Proto-Indo-European	-tis	Derives abstract/action nouns from verb roots

mastь — Etymology, Origin & Word History | Etymology Explorer