The story
Trademark; an abbreviation of "Water Displacement, 40th formula" or "40th attempt" used by Norm Larson of the Rocket Chemical Company of California, the oil's developer.
Etymology from Wiktionary, CC BY-SA 4.0.
WD-40
Today it means: A penetrating oil used as a universal solvent, a lubricant and to protect electric circuits from moisture. Full definition →