Knnthxbye is actually short for "KaNina No Thanks Goodbye." Kanina is a swear word originated from Hokkien.. A province in China. It's a very disrespectful phrase for ending a conversation abruptly with attitude.
Bob: Hey how bout a trip down to the gay pub tonite?
GayBill: ....... knnthxbye! *Storms off*
KaNina No Thanks Bye
fuckhead 1 : Hey! come on, let's go to CHC!
smart arse : WHAT??!?!?! KNNTHXBYE!!