slap sb in the face 打某人耳光， （喻）侮辱某人

【例句】
Her boyfriend cheated on her, so her slapped him in the face.
她女朋友劈腿了，所以她给了他一耳光。

His claim in public sounds he is innocent,but after the truth come out in the wash,it's totally a slap in the face.
他公开声称他是无辜的，但当真相大白时，那简直就是扇了一记耳光。