A corruption of the German weissager (a soothsayer or prophet). This, like the Greek sophism, has quite lost its original meaning, and is applied to dunces, wise only “in their own conceit.”
There is a story told that Ben Jonson, at the Devil's Tavern, in Fleet Street, said to a country gentleman who boasted of his landed estates, “What care we for your dirt and clods? Where you have an acre of land, I have ten acres of wit.” The landed gentleman retorted by calling Ben “Good Mr. Wiseacre.” The story may pass for what it is worth.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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