An abbreviation of a line from the intro to an
anime called “your lie in April” typical said as a sign of affection and love.
It stands for
Kimi
da yo kimi nanda yo. Oshiete kureta Kurayami
mo hikaru nara Hoshizora ni naru
And translates to
It was you, it was you! The one who made me realize; if we can make even the darkness shine, it will become a starry sky.