Source : Webster's Revised Unabridged Dictionary (1913)

Four-o'clock \Four"-o'clock`\, n.
   1. (Bot.) A plant of the genus {Mirabilis}. There are about
      half a dozen species, natives of the warmer parts of
      America. The common four-o'clock is {M. Jalapa}. Its
      flowers are white, yellow, and red, and open toward
      sunset, or earlier in cloudy weather; hence the name. It
      is also called {marvel of Peru}, and {afternoon lady}.

   2. (Zo["o]l.) The friar bird; -- so called from its cry,
      which resembles these words.