Where does “дрѣмати” come from?

дрѣмати (Old Church Slavonic) comes from Proto-Slavic drěmati, from Proto-Indo-European drem- — to run.

Ancestry of “дрѣмати”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	drěmati	to doze, to drowse, to slumber
2	Proto-Indo-European	drem-	to run

дрѣмати — Etymology, Origin & Word History | Etymology Explorer