Where does “czwartek” come from?

czwartek (Old Polish) comes from Proto-Slavic četvьrtъkъ, from Proto-Slavic četvьrtъ, from Proto-Balto-Slavic ketwirtas, from Proto-Indo-European *kʷetwr̥tós.

Ancestry of “czwartek”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	četvьrtъkъ	Thursday
2	Proto-Slavic	četvьrtъ	fourth
3	Proto-Balto-Slavic	ketwirtas	fourth
4	Proto-Indo-European	*kʷetwr̥tós	—

Every word from Proto-Indo-European *kʷetwr̥tós →

czwartek — Etymology, Origin & Word History | Etymology Explorer