There is a grade of plywood called WBP.  WPB is popularly supposed to stand for Water and Boil Proof.  This sounds phoney to me as “boil proof” makes “water proof” redundant.  So what does WBP stand for?

From here:

Exterior grade plywood (WBP - Weather and Boil Proof). This type of plywood can be used outside. Water-resistant adhesives are used and can resist a certain amount of moisture.

I jumped the gun on this one.  Seems there are other sites that insist on “water” in the acronymn (like DIY in the UK).  Is this a UK thing?  Maybe boiling water (the rating qualifying it for a certain number of hours in boiling water)?

It’s the “boil proof” that doesn’t ring true - I can’t think of any application for ply that would require it to be boiled and not fall apart.  Apparently exterior grade WBP uses the same glue as marine ply, but different wood.

Nevertheless, resistance to immersion in boiling water is used as a test for grading plywood; see here.
It’s not that unusual to use short exposures to more extreme conditions than would be encountered in actual use as an index for performance over longer periods under more reasonable conditions.  It’s impractical to test each lot of plywood by exposing it to actual weather for 5 years, but the ability to withstand a few hours of boiling offers some indication of its resistance to heat and moisture.

Given that some of the other tests involve immersion in water without boiling, and that, as I said, water treatment is more readily incorporated into a standard test than weather exposure, I would expect “Water and Boil Proof” to be the original source of the acronym, with “Weather...” a later substitution based on the fact that passing the water-and-boil test would provide a reasonable expectation of weather-resistance.