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)
(Hence being koo!)
Person B would then have licence to say: "Very Koo!"