To take a girl/boy away from your friend without him/her even knowing it is happening. The key is you have to do it right in front of him/her. If done behind his/her back you should search for backstabber.
John and Brian go to a bar with Marianna and Beth. John does not like beth even though she is the one setup for him. So John begins glumping Marianna infront of Brian. At the end of the night Brian is left with Beth at the bar while John and Marianna leave together.