More accurately, foolscap is 13.5 * 17 inches. It was more common before metrication, the nearest metric equivalent being A4. The name foolscap comes from the watermark originally used: the outline of a fool's cap.

Fools"cap` (?), n. [So called from the watermark of a fool's cap and bells used by old paper makers. See Fool's cap, under Fool.]

A writing paper made in sheets, ordinarily 16 x 13 inches, and folded so as to make a page 13 x 8 inches. See Paper.

© Webster 1913.