The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. (Zool.) The friar bird; -- so called from its cry, which
      resembles these words.
      [1913 Webster]