Cold Hearted
noun, な adjective
If you dilute feeling from someone, they have no feelings. They become cold hearted.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
彼は薄情者だし、どっちの味方かサッパリ分からないよ。
He is cold hearted and I have no idea whose side he’s taking.