Whereas 'Cockblocking' is the act of preventing someone from getting laid, the act of 'Cockswapping' is when the swapper purposely blocks the sexual attempts of a third party, only to move in after they've ruined everything for their own personal gain.
A cockswapper knowingly uses lies, money, or any other means to steal a potential sexual partner from someone else.
A cockswap can be performed by men and women, however is mostly used as a term by men, as women usually just call the offending party a 'slut'.
"I was talking to this girl, and then Bob told me she was ugly, and I could do better. Then, later that night, he cockswapped, and told her I called her ugly, and slept with her. What a prick!"