Learn English – Sarcastic Idioms for being slow to notice something

There will be a lot of such words and expressions (so I hope you'll resist the urge to select an answer prematurely). The one I'll propose is cut down. Here are a couple sample usages:

I didn't appreciate the way he cut me down during the meeting.
Maybe he cut down Susan during the meeting, but I don't think he did himself any favors.

The phrase is a shortened form of cut someone down to size, meaning "to put someone in their place."

It should be noted that the phrase cut down has several meanings, so you'll want to provide ample context to make it clear what you are talking about.

If you wanted to use a noun rather than a verb, you could try slap in the face. It is slang for "an insult."

Bob's remarks were a real slap in the face.

By the way, the word real in that example sentence means "serious," not "actual" (i.e., Definition #5, not Definition #1a, in this dictionary). Contrary to how it might be interpreted, that sentence doesn't mean that Bob was literally slapped in the face.

Sounds like you found the one you were looking for, but there are a number of other English idioms that also might work, depending on how exactly you are using them:

• "You can't unring a bell" (or perhaps "you can't unring that bell")
• "You can't unscramble an egg"
• Saying something you later regret is "putting your foot in your mouth"
• "Closing the barn door after the horse has bolted" is doing something too late.