I don't read message boards all that much, so this puzzles me. The closest I can think of for "un" is that maybe it's a cute little grunt or something, I dunno.
those make symbols show up in chat so people that frequent chat will know what they mean. i cant remember what (un) is which means i guess i never really use it in chat.
not cool. lame. stupid. un-anything. unnecessary, unattractive, undesireable, etc.
What you would say after hearing or seeing something that is offensive or disgusting. Should be said in a higher octave than your usual speech. The intention for the use of this word is to "UN" do it!
This seems to agree with the chat smiley that minty posted above!
Ohh, it makes so much more sense now. I kept imagining a little Asian girl going "un!" and making a fobby peace sign, which always seemed completely out of context.