Noun - one who overhears your conversation and inserts their own personal thoughts into said conversation without invitation. Often the hi-jacker's words will have no relevance to your conversation and has only served to take said conversation on a 20 minute tangent to nowhere.
Verbal piracy at its apex, bringing your conversation to its nadir.
"So I was discussing the bar scene at my alma-mater with Jen when Elle hi-jacked our conversation and we ended up talking about brownies for twenty minutes. Dammit."
-"Bro she is a conversation hi-jacker, you gotta be careful."