The Collaborative International Dictionary of English v.0.48:

Sunbonnet \Sun"bon"net\, n.
   A bonnet, generally made of some thin or light fabric,
   projecting beyond the face, and commonly having a cape, --
   worn by women as a protection against the sun.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sunbonnet
    n 1: a large bonnet that shades the face; worn by girls and
         women