Aristophanes coined a term meaning "an unrealistic or ridiculous utopia" in his satirical play "The Birds" in 414 ce. That term was later (much later-- 1800s) translated as "Cuckoo Cloud Land" or "Cloud Cuckoo Land." It's not much of a stretch to apply the beliefs of Cloud Cuckoo Land to the cuckoos themselves. Additionally, cuckolding referred to a woman's infidelity, often as an insult to her husband (unrelated to Cloud Cuckoo Land; most likely related to the cuckoos' hiding their eggs to be raised by other birds), again, painting the poor cuckoo in a less-than-flattering light.