The acronym for "This Conversation Is Over". A great and courteous way to put finality on all your text messages, conversations, etc. A great new way to get the last word!
<Jules: txt to Claire> What time do you want to meet at the restaurant tonight?
<Claire: txt to Jules> 6:30 p.m. TCIO
Jules now knows that Claire is done with this conversation and will probably be away from her phone. The conversation has been given finality.