Welcome aboard, Shiraz!
The ultimate etymology of Neshka appears to be
Agnes, meaing "pure" in Greek.
The Russian form of the name is
Agnessa from which scores of variations
evolved throughout the Slavic world (Neshka, Neska, Nesha,
Nessa, Nesya etc etc).
By the way
Daividh has a dear bearded aunt called Agnes-Neshka.