Where does “holkr” come from?

holkr (Old Norse) comes from Proto-Germanic hulkaz, from Proto-Indo-European ḱel- — to cover; to incline.

Ancestry of “holkr”, step by step

Step	Language	Word	Meaning
1	Proto-Germanic	hulkaz	cavity, hollow, recess; a hollow
2	Proto-Indo-European	ḱel-	to cover; to incline

holkr — Etymology, Origin & Word History | Etymology Explorer