The Collaborative International Dictionary of English v.0.48:

Blue bonnet \Blue" bon`net\ or Blue-bonnet \Blue"-bon`net\, n.
   1. A broad, flat Scottish cap of blue woolen, or one wearing
      such cap; a Scotchman.
      [1913 Webster]

   2. (Bot.) A plant. Same as Bluebottle.
      [1913 Webster]

   3. (Zool.) The European blue titmouse (Parus c[oe]ruleus);
      the bluecap.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bluebonnet
    n 1: low-growing annual herb of southwestern United States
         (Texas) having silky foliage and blue flowers; a leading
         cause of livestock poisoning in the southwestern United
         States [syn: bluebonnet, buffalo clover, Texas
         bluebonnet, Lupinus subcarnosus]
    2: a brimless dark blue Scottish cap with a flat top and a plume
       on one side [syn: Balmoral, bluebonnet]