if that isn't ...
1 Imagine you just sat through a three-hour movie with your significant other, and it was so boring that you had to struggle to keep your eyes open. When it finally ends, you say:
If that wasn't the most boring movie I've ever seen, I don't know what is!
In other words, What a boring movie!!!
The language used in this expression is pretty straightforward, so it won't require a lot of explanation. However, people will often just say the first part and leave off the second part. It's also common to put Well in front of it to make it sound more categorical.
Well if that wasn't the most boring movie!
You can use a superlative like the best, the worst, the ugliest, the most boring, the most interesting, etc. or you can just use a plain adjective, as in this meme, which is short for Well, If that isn't a pretty sight, I don't know what is.
When using the long form, don't forget to stress the verb to be at the end.
Boris is visiting his brother for the weekend and they start looking through the photo album.
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Bobby: Remember this one?
Boris: <gasp> How could I forget! My sixteenth birthday and that awful little mustache.
Bobby: And this is my Angie's wedding picture...
Boris: If she wasn't the most beautiful bride, (I don't know who was!)
2a In [1] above, we see this expression used to refer emphatically to the quality of something using an adjective. This phrase can also be used with nouns to emphasize that there's no way to deny what something is/was. For instance, say you and a friend are walking through an outdoor market and some guy is selling watches. Your gullible friend actually thinks he's getting a Rolex for $100, and you say:
Dude, if that ain't a fake, I don't know what is!
This is more or less the same as saying That's definitely a fake if I ever saw one!
In this meme, the commenter below identifies completely with first person in the meme: I like you, but I can't get into a relationship right now. Note that they say I don't know what is, but you can also say who is.
Again, the nuance here is: Yep, that's definitely me!
2b Sometimes, this expression is used with common idioms to point out perfect examples of what they imply. For instance, we have an idiom: to add insult to injury, which means to do or say something that makes an unpleasant situation even worse for someone.
So in this meme, it's bad enough that you got a fork in your tire, but to make it worse, the fork is flipping you off! (which means giving you the middle finger). If this ain't the perfect example of adding insult to injury, I don't know what is! 😜
3a The above expression looks very similar to another one: Well, if it isn't [person]! But they aren't used the same way and don't have the same meaning. This one is used when you encounter someone unexpectedly and it expresses surprise, with a nuance of «Look who's here!» Also, in this expression, we only use it, not that/this/these/those as in the above expression.
Like the expression above, you may hear a second part: ...as I live and breathe! which basically means «as alive as me», implying that you haven't heard from them or seen them in so long that you were starting to think they were dead. But as in the expression above, this part can be left off.
The men in this meme are from the series Outlander, in which they're arch enemies. The first man, Jamie, asks the second one if he's ready to fight, and the second man is caught by surprise because he thought he'd never see Jamie again, and yet here he is on the battlefield!
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Let's dance! is street talk for Let's fight!
3b This expression is most often used sarcastically when you feign surprise at seeing someone, but you're not surprised to see them at all. You knew they'd pop up sooner or later for some reason or another. When used sarcastically like this, it often has two or up to three wells; the more wells, the more sarcastic.
In this meme, the speaker sees a woman drinking — and probably already drunk, and upon finding her in this state, sarcastically reminds her that the last time she got drunk, she felt so bad that she said I'm never drinking again!, yet here she is (surprise, surprise!) DRINKING AGAIN!
As you can imagine, intonation is key in all the expressions on this page.
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When we throw people's words back in their faces, we often call men Mr. "[words]", and women Little Miss "[words]".