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When you want to say that someone doesn't know anything about a particular thing, do you say they don't know jack about it, or that they know jack about it?

I've seen it used both ways. Which is correct?

marked as duplicate by user240918, FumbleFingers, Nigel J, jimm101, Skooba Feb 14 '18 at 20:06

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That this and similar slang expressions set up the apparent paradox of x = not x has a particular fascination for linguists. The logic, however, is easy to grasp.

If jack is a quantity of absolutely no value, i. e. zero, then the difference between

He knows zero.

and

He doesn't know zero.

is basically zero.

Both expressions are "correct" in the descriptive sense that native speakers use both.

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