CIOIN

It's an oft-repeated tale that the grand dame of military computing, computer scientist and US Navy Rear Admiral Grace Hopper, coined the terms 'bug' and 'debug' after an incident involving Harvard University's Mark II calculator.

The story goes like this: On September 9, 1945, a Harvard technical team looked at Panel F and found something unusual between points in Relay 70. It was a moth, which they promptly removed and taped in the log book. Hopper added the caption: "First actual case of bug being found," and that's the first time anyone used the word bug to describe a computer glitch. Naturally, the term debugging followed.

This tale's got more bugs in it than Relay 70 probably ever had. For one thing, Harvard's Mark II came online two years after the date attributed to this story. For another, you don't use a line like "First actual case of bug being found" if the term bug isn't already in common use.  And although Hopper often talked about the moth in the relay, she did not make the discovery or the log entry.
The core facts of the story are true—including the date of September 9 and time of 15:45 hours—but that's not how this meaning of the word bug appeared in the dictionary. Inventors and engineers had been talking about bugs for more than a century before the moth in the relay incident. Even Thomas Edison used the word. Here's an extract of a letter he wrote in 1878 to Theodore Puskas: “'Bugs'—as such little faults and difficulties are called—show themselves and months of intense watching, study and labor are requisite before commercial success or failure is certainly reached.”

Word nerds trace the word bug to an old term for a monster—it's a word that has survived in obscure terms like bugaboo and bugbear and in a mangled form in the word boogeyman. Like gremlins in machinery, system bugs are malicious. Anyone who spends time trying to get all the faults out of a system knows how it feels: After a few hours of debugging, any problems that remain are hellspawn, mocking attempts to get rid of them with a devilish glee.
And that's the real origin of the term "bug."