Abbrievation for "Can't Talk Right Now" - commonly used online, like when someone is trying to IM you and you're really busy.
Or if you just don't like them, and are pretending to be really busy.
summer_girl19: Omg, you will NEVER believe what happened last night between me and Michael!
iluvorli_78: Ctrn - working on English paper.
summer_girl19: Ok, bye!