An MP3 song file that contains either a song different than what its name suggests, or a short sample of the song followed by noises or an anti-Napster message.
"Working after hours at the hardware store, the Fix brothers released their first batch of cuckoo's eggs in mid-June, transferring songs by Stephanie Fix to Napster and renaming them as songs by popular artists....The second batch of song files consisted of correctly named song files, but 30 or so seconds into each song, the brothers inserted an audio clip of the cartoon character Charlie Brown saying, 'Congratulations,' followed by a clip of Yosemite Sam saying, 'Looks like you goofed up somewhere,' followed by the sound of a chirping cuckoo that continued for the rest of the track."
Catherine Greenman, "Taking Sides in the Napster War," The New York Times, August 31, 2000
This phrase comes from the bizarre actions of the cuckoo bird, which lays an egg and then leaves it in the nest of another bird species. The egg is incubated and hatched by the other bird, which even feeds the resulting chick as if it were her own. The cuckoo chick will even go so far as to forcibly remove the other chicks from the nest, leaving itself as the sole beneficiary of the food provided by the mother bird.
Note, too, that there is such a thing as a "cuckoo egg program," which is a program that a malicious hacker leaves behind after he breaks into a system. Later, the operating system "hatches" the program automatically and the result is that the hacker gets enhanced system privileges the next time he breaks in. This was documented nicely in Clifford Stoll's book The Cuckoo's Egg.
The original cuckoo eggs were produced by the Cuckoo Egg Project: