Where does “ras” come from?

ras (Middle English) comes from Old Norse rás, from Proto-Germanic rēsō, from Proto-Indo-European reh₁s- — to flow, rush.

Ancestry of “ras”, step by step

Step	Language	Word	Meaning
1	Old Norse	rás	race, running; course, channel; company, host
2	Proto-Germanic	rēsō	a running; a course; a rush
3	Proto-Indo-European	reh₁s-	to flow, rush