The Collaborative International Dictionary of English v.0.48:

Swan's-down \Swan's"-down`\, or Swans-down \Swans"-down`\, n.
   1. The down, or fine, soft feathers, of the swan, used on
      various articles of dress.
      [1913 Webster]

   2. A fine, soft, thick cloth of wool mixed with silk or
      cotton; a sort of twilled fustian, like moleskin.
      [1913 Webster]

   Swan's-down cotton. See Cotton flannel, under Cotton.
      [1913 Webster]