Berserker means “bear shirt” and it is what those guys wore-a bearskin shirt/tunic-their battle lust/insanity was the result of them having consumed beer and hallucinogenic plants before battle-they were on the equivalent of a bad acid trip...
Their afterlife also beckoned.
Valhalla promised daily beer and battle until the final “Reckoning”, where chosen warriors went to finality.
Northmen also raided Holland, pillaging the [coastal] Frisans—who I thought were part of the Northmen—but must be wrong.
Frisans speak a different Dutch dialect—related to English.