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!
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It means "Can't Talk Right Now"
Someone:hey
*pretend you're busy*
Me: hey sorry ctrn