A Finnish version of foo or LOL, a default answer when someone pastes URL, but it's not enough for 'lolling'. Also something that is said when joining the channel or just at a random time as in the example below. Correct answer to köö is ... köö.
Köö can be also used as a part of a word, such as "öököö", meaning OK.
Captain Kirk's name is also sometimes spelt "Köök".
In fact, köö can mean anything.
Someone with handle "Kissa" invented the word 'köö' and it was first used in the IRC-channel #comrades in Quakenet, from where it started to slowly spread around. #comrades is still the main place of 'köökötys' (=kööking)