the.com/cuckoo clock

means A clock that marks the hour with a mechanical bird popping out to call 'cuckoo' the corresponding number of times.

from Straightforwardly named for the real cuckoo bird, whose simple two-note call the clock's bellows and pipes imitate. The clocks trace to Germany's Black Forest region, where 17th–18th century clockmakers built cheap wooden-geared clocks for local trade; the cuckoo mechanism became the region's signature novelty and the thing that made these clocks famous worldwide. 'Cuckoo' meaning 'crazy' comes from the same bird — its insistent, repetitive call — but that's a separate branch of the word's life, not the clock's reason for being.