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!