This is the sort of burning question that lots of people wonder about for some reason.
To solve the mystery, the best place to go is the WD-40 Web site. If you dig around a little, you find the following fascinating quote:
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.
Now don't you feel better knowing that?