The product began from a search for a rust preventative solvent and degreaser to protect missile parts. WD-40 was created in 1953 by three technicians at the San Diego Rocket Chemical Company. Its name comes from the project that was to find a "water displacement" compound. They were successful with the fortieth formulation, thus WD-40. The Corvair Company bought it in bulk to protect their atlas missile parts.

It's a "Water Displacing" (WD) lubricant.

The "40" comes from the fact that it was the 40th attempt at making the product that was finally successful.

Water Displacement ..40 th Experiment ...

It is the name of a lubrication product