What does "Forget about it" mean?
I only know one of its meanings that is used to say that something that happened was not important or not something to worry about.
"I'm sorry I was late." "Forget about it. (=It's OK.)"
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But how about these examples. I can't get it.

- Today's lunch was fantastic. Forget about it. It was unbelievable.
- I cannot believe how expensive everything is these days. Forget about it. Prices are through the roof.

Question

What does "Forget about it" mean?
I only know one of its meanings that is used to say that something that happened was not important or not something to worry about.
"I'm sorry I was late." "Forget about it. (=It's OK.)"
--------------------------------------------
But how about these examples. I can't get it.

- Today's lunch was fantastic. Forget about it. It was unbelievable.
- I cannot believe how expensive everything is these days. Forget about it. Prices are through the roof.

Answer

In both examples, "forget about it" is acting as an informal intensifier. It's a way of saying "really, really fantastic" or "how extremely expensive." You usually only hear it used this way in the northeastern US around the New York City and Philadelphia metropolitan regions.

Answer

In American English, “Forget about it!” can also have a meaning similar to “Wow, incredible!”

So if somebody says: “This pizza, *chef’s kiss* forget about it! You won’t find a better pizza anywhere in town.

It may often be pronounced “fuhgeddaboudit!”

Answer

There's an entire scene about this in the movie Donne Brasco.  If you google   Donnie Brasco Forget about it     you'll see that it can mean many different things.