A few years back, I read John Wyndham's THE MIDWICH CUCKOOS, on which the cool flick VILLAGE OF THE DAMNED was based. And suddenly I had a mild revelation. The book is about some sort of extraterrestrial infiltration. Every fertile woman in a small English village becomes inexplicably pregnant and delivers an offspring who looks so much like the other Children that they might as well be clones. And they have a group mind, and things in general go as badly as you might expect.
Now, the title THE MIDWICH CUCKOOS refers to the unkind practice the European Cuckoo has. This bird stealthily flings the eggs from another bird's nest out to their deaths, then lays its own eggs there. And while the unsuspecting mother raises the Cuckoo as if it were her own, the heartless Cuckoo mother is off hanging around the beach, getting drunk every night and going home with strange.. okay, that's getting too anthropomorphic, I guess. Anyway, look at the poor wren in that picture, stuck with struggling to feed a baby several times bigger while her own dim little one-watt brain tries to figure out what went wrong with her life.
As a child, I had always associated the adjective "cuckoo" with those pointless clocks that had a mechanical bird pop out and chirp at each hour before snapping back in. But now it seemed that the origin might have come from parents telling their misbehaving child, "You're not one of ours, you're a little cuckoo." Much the same way they used to tell their brats they had been left by Gypsies and their real child stolen.