A collective term for a hoodlum attempting to gain status in the family.
The outsider is gradually brought into a family through a series of jobs, starting with small crimes, then be present at bigger crimes, then being an accomplice, then performing the crime.
It is not an official position in La Cosa Nostra but more of an umbrella term for people doing jobs who are "on the outs" with a family but it also is used to refer to civillians from time to time.
Outsiders usually perform simple tasks but do a lot of "heavy lifting" eg. packing stolen goods into a truck, threatening a shopkeeper to earn some protection money and then pass their percentage to who they report to, they are called "outsiders" because they are "on the outs", family business dealings are not discussed with them, it's only once they have worked their way up to associate status that they are given the chance to make their bones.
Any hit for an outsider often puts them in a position of being a disposable hitman and they may not survive to collect their pay for the job or they may very well be executed by the person who requested them to perform the hit in an attempt for the person who brokered the hit to tie up all loose ends.
When somebody "makes their bones" a term used in La Cosa Nostra for the first murder one commits under orders, often a soldato will take a new associate with them to "see how it's done", after that, the associate shall have to be an accomplice with something simple like cleaning up the scene of the hit, moving a body, then it would be something like assisting in executing somebody and finally, to be given their own hit to perform.