'WD-40 literally stands for Water Displacement, 40th attempt. That's the name straight out of the lab book used by the chemist who developed WD-40 back in 1953. The chemist, Norm Larsen, was attempting to concoct a formula to prevent corrosion-a task which is done by displacing water. Norm's persistence paid off when he perfected the formula on his 40th try.
http://inventors.about.com/od/wstartinventions/a/WDFourty.htm