From WordNet (r) 2.0 (August 2003) [wn]:
foolscap
n : a size of paper used especially in Britain
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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.
[1913 Webster]