A size of paper historically used for printing and writing, roughly 13 by 17 inches, or a tall pointed hat traditionally worn by a court jester.
Named from the watermark design of a fool's cap with bells that appeared on foolscap paper made in the 17th century; the term evolved to describe both the paper and the jester's hat.
The paper size got its name from a watermark depicting a jester's hat—it's a rare example of a commercial product named after a permanent mark on it, and it shows how completely different meanings of 'foolscap' developed from a single historical image.
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