Kuuki Yomenai = K.Y.(Adj,,n.)
When I first heard it I thought of something dirty. Don't lie, you know you did too!
It literally means "cannot read the air"
While you are living in Japan you will find this is a commonly used phrase. It is NOT ONLY used by young people. It is referring to a person who is oblivious to his current surrounding, and may say or do something insensitive or inappropriate.
This has a negative connotation of ignorance.
Synonymous words would be : Dense or Thick-headed.
Another variation is C.K.Y.
C is for Chou = super, really, extremely