Usually used when a friend brings along another "friend" into your group to hang out. you and the other people normally in your crew are not to sure about the credentials of this "new friend". you ask around to your buddies if anyone knows this new guy (most likely already forgetting this new guys name) hence calling him "donnie brasco". as not to mention to much inside info/gossip as no one in your group is aware on who or what this guy is all about.
"hey Jimmy... whats up with "donnie brasco" over there?"
"i dunno Tommy.. Frankie brought him by tonight. you know anything about this guy?"