Source : Webster's Revised Unabridged Dictionary (1913)

Foolscap \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}.

Source : WordNet®

foolscap
     n : a size of paper used especially in Britain