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 gāndiē 'godfather', 干妈 gānmā 'godmother'). Common in old novels, business / 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 4th-tone 'dry/related-by-name' — written 乾 in traditional. Don't confuse with the modern slang 干儿子 (gàn'érzi, 'fucked-up kid' or worse) where 干 is the vulgar verb. Tone differs (gān vs gàn) and so does context — the kinship sense is gān, first tone.