干儿子

Different from the legal adoption sense of English 'adopted son'. A 干儿子 keeps his birth family and surname; the relationship is a sworn social bond often sealed with a small ceremony, gifts, and address-term changes (干爹 'godfather', 干妈 'godmother'). Common in old novels, business and triad fiction (where powerful men collect 干儿子 as loyal followers), and family customs where parents 'gift' a frail child to a stronger godparent for protection.

干 here is the first-tone 'dry/related-by-name' sense - written 乾 in traditional. Don't confuse it with the vulgar verb reading 干 (fourth tone). Tone differs (gān vs gàn) and so does context - the kinship sense is gān, first tone.