Wiktionary
make one's bones
vb. (context idiomatic English) To commit a murder in order to be admitted to a criminal gang.
Wikipedia
Make one's bones
To make one's bones is an American English idiom meaning to take actions to establish achievement, status, or respect. It stems from, and is a corruption of, the idiom "establish one's bona fides".
Although the idiom appears to have originated in the United States criminal underworld it has since migrated to more popular and less sinister usage; including law enforcement, legal, and religious discussions.