WD-40's name records what its developers were trying to accomplish and how many attempts it took. The letters refer to water displacement; the number marks the fortieth formula. According to the company's own history, the successful version emerged in 1953 from a small Rocket Chemical Company laboratory in San Diego.
The original problem belonged to industry rather than a household toolbox. Aerospace contractor Convair used the product to protect the Atlas missile's outer surface from rust and corrosion. Employees also found uses for it at home, helping reveal a wider market.
Aerosol cans reached San Diego store shelves in 1958. That history explains the familiar name without turning it into a promise that one product is right for every repair. The label preserves a particular engineering goal and the persistence needed to reach it.
Sources: WD-40 company history