The Collaborative International Dictionary of English v.0.48:

Whitecap \White"cap`\, n.
   1. (Zool.)
      (a) The European redstart; -- so called from its white
          forehead.
      (b) The whitethroat; -- so called from its gray head.
      (c) The European tree sparrow.
          [1913 Webster]

   2. A wave whose crest breaks into white foam, as when the
      wind is freshening.
      [1913 Webster]

   3. A member of a self-appointed vigilance committee
      attempting by lynch-law methods to drive away or coerce
      persons obnoxious to it. Some early ones wore white hoods
      or masks. [U. S.] -- White"cap`, v. -- White"cap`per,
      n.
      [Webster 1913 Suppl.]