Contraction of Ok, thanks, bye.
Used when you want someone to shut up (you're not exactly trying to be polite,) yet it's not considered as an insult like STFU. Also it can be used when answering a stupid n00b question, or after scolding someone.
I'm not arguing anymore. kthnxbai.
Stop that or I'll ban you. kthnxbai.
^ above isn't as insulting as STFU N00B!