Webster's Revised Unabridged Dictionary · 1913

Four-o'clock

Four-o'clock , noun

1.
(Botany) 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.
(Zoology) The friar bird; -- so called from its cry, which resembles these words.