E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Adoption by arms. An ancient custom of giving arms to a person of merit, which laid him under the obligation of being your champion and defender.
Adoption by baptism. Being godfather or godmother to a child. The child by baptism is your god-child.
Adoption by hair. Cutting off your hair, and giving it to a person in proof that you receive him as your adopted father. Thus Boson, King of Arles, cut off his hair and gave it to Pope John VIII., who adopted him.