Where does “Szczur” come from?

Szczur (Polish) comes from Proto-Slavic ščurъ, from Proto-Indo-European skewh₂- — to lash, to swing, to buzz.

Ancestry of “Szczur”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	ščurъ	playful, mischievous, perky; void, idle; type of...
2	Proto-Indo-European	skewh₂-	to lash, to swing, to buzz

Szczur — Etymology, Origin & Word History | Etymology Explorer