Where does “ча́ю” come from?

ча́ю (Russian) comes from Russian ча́ять, from Proto-Slavic čàjati — to expect, to wait.

Ancestry of “ча́ю”, step by step

Step	Language	Word	Meaning
1	Russian	ча́ять	—
2	Proto-Slavic	čàjati	to expect, to wait