Where does “buk” come from?

buk (Old Polish) comes from Proto-Slavic bukъ, from Proto-Indo-European bʰeh₂ǵos — beech.

Ancestry of “buk”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	bukъ	echo, noise of splashing water; beech
2	Proto-Indo-European	bʰeh₂ǵos	beech

buk — Etymology, Origin & Word History | Etymology Explorer