Is the idiom "in forever" grammatically correct?

"I haven't seen you in forever."

It seems as though it should be:

"I haven't seen you for ever."

For some reason this bothers me, and I don't know why. It seems like there is either a double preposition, or a preposition with no object, or maybe it's just my imagination because EVERYONE seems to use this expression except for me.

Do any educated people out there know the answer to this?
Hello Anon,

1. I haven't seen you in forever.

I would take this as a jocular variant of:

2. I haven't seen you in an age [or some similar time-phrase].

For "an age", which is already hyperbolic, the speaker substitutes "forever", which piles hyperbole on hyperbole, and raises it to the power of impossibility.


3. It took me forever.

(Clearly it didn't.)

All the best,

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I understand that it is a hyperbole, but I want to know if it is grammatically correct.

Is the expression itself, "... in forever" grammatically correct. I am assuming it is, considering the word "forever" can be a noun, but it is most commonly an adverb and therefor it can be the object of the preposition. In this case, the preposition is the word "in"... but for some reason, it does not sound right to me.
Grammatically, I think it can only be "correct", since "forever" can be a noun, as in:

1. It took me forever.

(Cf. "It took me an hour".)

Semantically, you might say that it's not quite right; but presumably that slight not-quite-right-ness is the source of its appeal (for those who use it).

Best wishes,

Is the term “ in forever” grammatically correct?
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which is grammatically correct "friends forever" or "forever friends"