Where does “пискав” come from?

пискав (Bulgarian) comes from Bulgarian пи́сък, from Proto-Slavic *piskъ — scream?, squeak?.

Ancestry of “пискав”, step by step

Step	Language	Word	Meaning
1	Bulgarian	пи́сък	scream, shriek, screech, cry a sharp, high-pitched cry made by a person or animal
2	Proto-Slavic	*piskъ	scream?, squeak?