a slap in the face是“打脸、一记耳光”，引申为侮辱。

【例句】
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。

They felt that it was a slap in the face when their gift was returned unopened.
他们的礼物原封不动地被退了回来，他们感到这是一个羞辱。