Where does “biskajski” come from?

biskajski (Polish) comes from Polish -ski, from Old Polish -ski, from Proto-Slavic -ьskъ, from Proto-Indo-European -iskos — Characteristic of, typical of, pertaining to.

Ancestry of “biskajski”, step by step

Step	Language	Word	Meaning
1	Polish	-ski	forms surnames; forms adjectives
2	Old Polish	-ski	forms adjectives
3	Proto-Slavic	-ьskъ	-ic, -ish
4	Proto-Indo-European	-iskos	Characteristic of, typical of, pertaining to

Every word from Proto-Indo-European -iskos →Every word from Polish -ski →Every word from Proto-Slavic -ьskъ →