Where does “часок” come from?

часок (Russian) comes from Russian час, from Old East Slavic часъ, from Proto-Slavic časъ, from Proto-Balto-Slavic keʔs-, from Proto-Indo-European k(ʷ)eh₁s-.

Ancestry of “часок”, step by step

Step	Language	Word	Meaning
1	Russian	час	hour, o'clock, time, time of day; hours, o'clock;...
2	Old East Slavic	часъ	time; hour; happiness, luck
3	Proto-Slavic	časъ	time
4	Proto-Balto-Slavic	keʔs-	—
5	Proto-Indo-European	k(ʷ)eh₁s-	—

Every word from Proto-Indo-European k(ʷ)eh₁s- →Every word from Proto-Balto-Slavic keʔs- →Every word from Proto-Slavic časъ →