What does WD mean in WD-40?

Water Displacement~~WD-40 was originally developed in 1953 by the Rocket Chemical Company, as a rust preventative solvent. Their first customer, Convair, used it to protect the outer skin of the Atlas Missile, America's first nuclear intercontinental ballistic missile. The formula was the 40th attempt at developing the suitable water displacement product, thus the name "WD-40". The original ICBM-protecting formula remains unchanged to this day, even if its uses are somewhat more mundane.