Where did the expression "You don't know Jack" come from? Who was Jack? A person? Something to lift up a car? A knife? You seem able to find the origin of just about everything.

Where did the expression "You don't know Jack" come from? Who was Jack? A person? Something to lift up a car? A knife? You seem able to find the origin of just about everything.

— Donna Lee N., Jacksonville

That's an abbreviated or Bowdlerized version of the complete expression, which is one you may hear in movies or on television these days but will not read in this family newspaper, namely, "You don't know Jack S—-." And we're not talking the lead character from "Pirates of the Caribbean" or someone named after a crouching position.

The expression is unusual in that it can be used with either of the two key words omitted and still make sense. And both nouns, the vulgar one and the other one, have long been catch-all slang terms in English, that is, they are used to represent almost anything (think of your own examples).

One of the more provocative ideas we turned up on this one has to do with the use of "jack" as slang for money. This goes back in this country as far as the 1700s and continued right on through the 20th century. This is a use of "jack" that includes the idea of something nearly worthless or minimized ("It's not worth jack").

The effect of putting the two words together is to intensify both in a colorful, informal way, in the way of such a pairing as "doodly squat." Notice that one works with either of the words omitted, too.

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