fóstbróðir (ON) noun

A man allied with another in a relationship of mutual rights and obligations. The relationship did not require kinship, and was established through joint upbringing (or through some ritual as suggested by sagas). In GuL, a fóstbróðir could receive compensation if the other was killed.

foster-brother ONorw GuL Løb, Mhb Refs:

Fritzner s.v. fóstbróðir; KLNM s.v. fostbrorskap