A cuckold. The cuckoo occupies the nest and eats the eggs of other birds; and Dr. Johnson says “it was usual to alarm a husband at the approach of an adulterer by calling out `Cuckoo,' which by mistake was applied in time to the person warned.” Green calls the cuckoo “the cuckold's quirister” (Quip for an Upstart Courtier, 1620). This is an instance of how words get in time perverted from their original meaning. The Romans used to call an adulterer a “cuckoo,” as “Te cuculum uxor ex lustris rapit ” (Plautus: Asinaria, v. 3), and the allusion was simple and correct; but Dr. Johnson's explanation will hardly satisfy anyone for the modern perversion of the word.
The cuckoo, then, on every tree, Mocks married men; for thus sings he, Cuckoo! Cuckoo! cuckoo! O word of fear, Unpleasing to a married ear!
Shakespeare: Love's Labour's Lost, v. 2.
(A). A watch or clock. The French have the same slang word coucou for a watch or clock. Of course, the word is derived from the German cuckoo-clocks, which, instead of striking the hour, cry cuckoo.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894