1) a person who is not a loyal individual. a person who talks shit about person a when he/she is in the company of person b and talks shit about person b when he/she is in the company of person a. a person who is superficially a good friend.
2) also can be used to describe actions that are of a disloyal nature.
3) also can be used an exclamation when an action is of a disloyal nature
1) She's always doing things like that behind her so-called friends' backs - she's such a chameleon.
2) She definitely lied...that was so chameleon.
3) (a friend just threw water in your face and you exclaim) "CHAMELEON!!"