Where does “tachy-” come from?

tachy- (English) comes from Ancient Greek ταχύς, from Proto-Indo-European dʰn̥gʰ-, from Proto-Indo-European dʰengʰ- — to cover; to overcast.

tachy- (English): fast

Definitions

Step	Language	Word	Meaning
1	Ancient Greek	ταχύς	swift, quick, fast, rapid; In a short time: soon
2	Proto-Indo-European	dʰn̥gʰ-	—
3	Proto-Indo-European	dʰengʰ-	to cover; to overcast

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