A third party who "ninjas" their way into a conversation they were not originally a part, usually with a comment which mean they have been eavesdropping for an extended period of time, like a ninja striking in the shadows.
Jackie just pulled a conversation ninja. I didn't even notice her before she dropped that comment about her Lexus while we were talking about luxury cars.
One makes a single random comment in someone elses conversation, which ends up killing it and making it awkward for everyone else. This is similar to a ninja swiftly killing a target then exiting.
Guy 1: Hey, do you see that really hot chick over there?
Guy 2: Hell yea, Id hit that all night!
Conversation Ninja: I screwed her and gave her herpes *disappears*
Guy 1: Oh... Im gonna go now.