https://www.etymonline.com/word/cuckold
The more modern use of the term "cuckoo" often means "crazy", often in a funny or cartoonish fashion, but it originally meant Cuckold, referring to how many species of Cuckoo Bird lay their eggs in the nest of other birds, and often once the cuckoo bird chick hatches, it will kill all the other chicks in the nest, whether they are eggs or hatched, or even if they are much bigger than them (because even if they are several times bigger than the cuckoo chick they don't understand that the cuckoo chick is killing them and don't defend themselves), and the original parents will not do anything to stop it and just continue to feed and raise the cuckoo bird that killed all their own offspring even if it grows to look nothing like its own species, because they don't understand what's happening or instinctually have a way to deal with it.
https://www.youtube.com/watch?v=lgIL8dbSMUA