Where does “[[čiest]], [[čěst]]” come from?

[[čiest]], [[čěst]] (Old Czech) comes from Proto-Slavic čęstь — part.

Ancestry of “[[čiest]], [[čěst]]”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	čęstь	part

[[čiest]], [[čěst]] — Etymology, Origin & Word History | Etymology Explorer