The first time I heard the expression "KY" was in a train full of junior college women playing with their cell phones, twiddling their bangs, and squealing at the top of their lungs--oblivious to all around them.
In the midst of this estrogen-fueled chaos, a woman in front of me yelled to her friends, 「あの人ってめちゃKY！」(ano hito tte mecha KY!). This roughly translates as: "That person is really clueless!" The group exploded in laughter and agreement.
In Japanese, "KY" has nothing to do with K-Y Jelly. It is a combination of "ku-ki"（空気）, which means "air" or "atmosphere" and is the K. The Y is "yomu"（読む）, which means to "read." The Y is usually used in its negative potential form--読めない ("yomenai"), meaning "can't read."
Thus, KY is slang for someone who can't read the situation, doesn't get it, is completely out to lunch.
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