To form a deep and lasting friendship; to become sworn brothers or sisters; the phrase originates from the [[易经|易經|yi4 jing1]] comparing the strength of a bond to metal and its fragrance to orchids

An intimate friendship or sworn brotherhood, or friends who are as close as brothers

(old) records listing the ancestry of sworn brothers, with each brother keeping a copy of the list

to have a deep and lasting friendship; to be as close as brothers or sisters

a list of names and details of people who have entered into a bond of sworn brotherhood