From Proto-Albanian *baitši, from Proto-Indo-European *bʰeydʰ- (modern be). According to Lubotsky, related to an Illyrian-derived toponym Besoi. Cognate to Old Irish bés according to the Trumper, proto-Celtic word might derive from Proto-Indo-European *bʰeydʰ- instead, thus rendering the Albanian besë as a cognate and explains the relationship between them as an Albanian/Celtic isogloss..
besë f (definite singular besa)
- pledge, word of honor
- (old) vow to avenge a murder
- (old) protection during a truce or when hosted as a guest, aegis
- (colloquial) religious faith, creed
- ^ Trumper, John. "Some Celto‑Albanian isoglosses and their implications." Structuring Variation in Romance Linguistics and Beyond. In honour of Leonardo M. Savoia (2018).