Heil öll!

Snorri Sturluson wrote this curious
stanza, consisting of no less than
16 sentences in 8 lines. He called
the metre "Sextánmælt"; "Sixteen-spoken".

Vex íðn. Vellir roðna.
Verpr lind. Þrimu snerpir.
Fæsk gagn. Fylkir eignask.
Falr hitnar. Seðsk vitnir.
Skekr rönd. Skildir bendask.
Skelfr askr. Griðum raskar.
Brandr gellr. Brynjur sundrask.
Braka spjör. Litask örvar.

My question to you:

What did Snorri mean by "Skelfr askr." ?

Kveðja,
Haukur

P.S. DeepStream is quite close to solving
the runic puzzle. I will post a solution
after the weekend - along with a new puzzle.