To "make one's bones" is an American English idiom meaning to take actions to establish achievement, status, or respect.
[1] It is an idiomatic equivalent of "establish[ing] one's bona fides".
[2] Although the idiom appears to have originated in the United States criminal underworld,[2][3] it has since migrated to more popular and less sinister usage;[4][5][6][7] such as discussions of various professions and occupations including law enforcement personnel,[8] the legal profession,[9][10] and journalists.
[11] The idiom was popularized in the 1969 book The Godfather and its 1972 movie adaptation, for instance when Sonny says "I 'made my bones' when I was nineteen, the last time the family had a war", and when Moe Greene says "I'm Moe Greene!
As in these examples, in organized-crime usage the phrase refers to establishing one's credibility by killing someone.