Pronounced ko-wai-ku-nai, it's Japanese for "not cute" (with the emphatic sentence-ending particle "yo" which can be freely substituted for "zo" or "ze") and is used to mean just that. However, it does not pertain to looks alone. One's thinking or behaviour may also be deemed "not cute."
Note that it is not often used in a sentence, but as a standalone phrase, in and of itself.
Most commonly used by a group of friends calling themselves the Alliance, founded in Upper Marlboro, MD.
James steals a game from EB Games. Tina witnesses this event, turns to her friend and simply states "kawaiku nai yo."