1) One who faslely betrays the confidence of a childhood friend, trusted ally, or former-partner.
2) One who, without reason or provocation, lies and back-stabbs a loyal freind who has always helped them and been there for them.
From the play "Hamlet", in which Prince Hamlet's boyhood friends, Rosencrantz & Guildenstern, pretend to be loyal to Hamlet so they could spy on him for the King.
"Tommy is a dirty Rosencrantzer: He likes to betray his friends. He thinks that by lying to soil your reputation, his will look cleaner."