Definition of bonnet-headed in English:

bonnet-headed

adjective

rare
  • 1Having a head or top resembling a bonnet.

  • 2Architecture
    (of an arch, window, etc.) having or consisting of an arch in which the angle of the return is more splayed than that of the jambs, considered to resemble the brim of a bonnet.

Origin

Mid 19th century; earliest use found in Cottage Gardener.