Where does “strast” come from?

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

Ancestry of “strast”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	strastь	emotional burden, pain; passion, strong emotion;...
2	Proto-Slavic	-tь	Deverbative, creates nouns denoting act, state,...
3	Proto-Indo-European	-tis	Derives abstract/action nouns from verb roots

Every word from Proto-Indo-European -tis →

strast — Etymology, Origin & Word History | Etymology Explorer