In certain countries such as ireland, this is used to mean Taking The Piss. This basically means having a laugh or joking.Saying this to someone is you telling them that you didn't mean any harm by it.
Person 1: I broke your lamp
Person 2 : I trusted you!
Person 1: relax, I'm just TTP
Person 2: thank god Person 1 : you can have it back, catch+
*smash*
Person 1: oops...