Where does “Langjökull” come from?

Langjökull (Icelandic) comes from Icelandic jökull, from Old Norse jǫkull, from Old Norse jaki, from Proto-Germanic jekô, from Proto-Indo-European h₁yeg- — ice, icicle.

Ancestry of “Langjökull”, step by step

Langjökull traces back through two separate lines of ancestry.

via Icelandic jökull

Step	Language	Word	Meaning
1	Icelandic	jökull	glacier
2	Old Norse	jǫkull	icicle; ice; glacier
3	Old Norse	jaki	broken ice, icefloe
4	Proto-Germanic	jekô	piece of ice; hoarfrost
5	Proto-Indo-European	h₁yeg-	ice, icicle

via Icelandic langur

Step	Language	Word	Meaning
1	Icelandic	langur	long; "only used in set phrases"
2	Old Norse	langr	"; long, far, distant; long
3	Proto-Germanic	langaz	long
4	Proto-Indo-European	dlongʰos	long

Every word from Proto-Indo-European h₁yeg- →Every word from Proto-Germanic jekô →Every word from Old Norse jaki →