Forty years ago I worked with a founder of Peterson Packing and
Filling of Hegeler, Illinois.  They were a contract aerosol filler
and one of the products they did was WD-40.  Paul is correct about
it being a water repellent.  WD stands for "Water Dispersal", and
40 is the fortieth formula they tried.

Although it is an internationally known product, back then WD-40
was based in San Diego and had only a few employees.  Almost of
everything was contracted out.

Jerry Wagner

 [ There's more on the origin of the name here:
 [ https://www.wd40company.com/our-company/our-history/
 [ --Jody