slap 作动词，是“打耳光”；作名词，表示“耳光、拍打”的意思。所以，a slap in the face 就是说“对自己的侮辱、打击、打脸、一记耳光”。

【例句】
His decision to leave the company to work for competitors was a slap in the face.
一个同事离职，去了竞争对手的公司，也可以算打脸。

It was a slap in the face for her parents when she ignored their advice and gave up her job.
她无视父母的忠告，放弃了自已的工作，是对她父母的公然侮辱。