The Collaborative International Dictionary of English v.0.48:

Leghorn \Leg"horn\ (l[e^]g"h[^o]rn), n.
   A straw plaiting used for bonnets and hats, made from the
   straw of a particular kind of wheat, grown for the purpose in
   Tuscany, Italy; -- so called from Leghorn, the place of
   exportation.
   [1913 Webster]

WordNet (r) 3.0 (2006):

leghorn
    n 1: a stiff hat made of straw with a flat crown [syn: boater,
         leghorn, Panama, Panama hat, sailor, skimmer,
         straw hat]