Completely changing the way you act to someone with little or no reason for doing so. A complete 180 (reverse direction) personality shift towards another person, leaving the other person completely confused at the 180-persons conviction for being "that way".
John and Samantha were really hitting it off and John was telling me he was really starting to like Samantha. Then she did a complete personality 180 on him and quit returning/answering his phone calls and she completely ignored him at the party. What a cunt.