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I saw the following sentence and I cannot understand its exact meaning.

Since when is W equal to PV ?

I do not understand the combination of "since + when".

Is it also formal or informal?

By the way W means work, P means pressure and V means volume. Basically, it is a mathematical formula.

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  • Can you add what context this is in?
    – Joe Kerr
    Nov 21 '20 at 19:31
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    It's informal, expressing disbelief: "Since when did you have a Mustang?" The formula should involve the change in volume. Nov 21 '20 at 19:32
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Although the answer by @Colin Fine is correct, it doesn't completely cover the usage.

Example

John: Bill, your grammar isn't good, I think you should study more.

Bill: Huh! Since when are you an expert on grammar? I get better marks than you do.

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  • Actually, that is more suitable for the context that I read, So I will accept this as a correct answer. Thanks @chasly
    – Malachi
    Nov 22 '20 at 8:15
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It's a common informal way of saying "I don't believe that is correct".

On the surface it is saying "As far as I know, X is correct. How long has what you said been right?"

It is slightly combative, but less than saying directly "That's wrong".

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    There are other uses of "since when"/
    – Lambie
    Nov 21 '20 at 20:32
  • I would say it is more insulting than saying ""You're wrong" Nov 21 '20 at 21:59
  • Maybe, @chasly-supportsMonica. Perhaps it depends on the social context
    – Colin Fine
    Nov 21 '20 at 22:15

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