It is an acronym that stands for "Just Kidding...... Not really". This saying means that you weren't actually kidding. It can be used in many ways.
1. An easy way to get out of an insult.... but adding a bitchy twist.
2. It basically means that you meant what you said.
3. It adds humor to an embarrassing thing you just admitted.
(often used on facebook and texting.)
ts;nr is an acronym for, "Too Short; Not Relevant".
Used to identify the fact that someone is too lazy to put together a response that is relevant to what you have asked, but still wants to post something to seem relevant.