Unfaithful
Infidelity
noun, な adjective, の adjective
When you do not have chastity, it's probably because you were unfaithful to your promise to whoever you told you'd stay chaste to.
ふてい
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
妻が不貞をはたらいたので、離婚しました。
My wife was unfaithful so we got divorced.