…her crew helped to popularize the terms bug and debugging. The Mark II version of the Harvard computer was in a building without window screens. One night the machine conked out, and the crew began looking for the problem. They found a moth with a wingspan of four inches that had gotten smashed in one of the electromechanical relays. It was retrieved and paster into the log book with Scotch tape. “Panel F (moth) in relay,” the entry noted. “First actual case of bug being found.” From then on, they referred to ferreting out glitches as “debugging the machine”
From: The Innovators: How a Group of Inventors, Hackers, Geniuses and Geeks Created the Digital Revolution by Walter Isaacson