Connor MoranOK, here's my fave.
The UK was working on a secret project to build armored vehicles capable of carrying field artillery, but for security reasons they didn't want the name of the project alone to give it away.
Using basically the "homework folder" approach to infosec, they planned to refer to the vehicle as a "water carrier." The commander didn't like having the initials be "W.C.," so they came up with a synonym.
Tank.