What does the "p" in p-coat stand for?
The name of these hip-length, double-breasted, wool military coats has been relaxed and now they are usually called pea coats or pea jackets.
The jacket dates back to the 18th century and is typically a cheap-to-produce, tough, warm coat which keeps it a popular choice for the military, particularly the U.S. Navy and Coast Guard.
But don't jump to conclusions. The "p" originally associated with these heavy-duty coats didn't refer to the soupy weather from which the coat offered protection. Rather, it referred to a very heavy, coarse, strong-twilled fabric sometimes called pilot cloth (or p-cloth), of which the jackets were originally made.
We've found an interesting page of Navy Uniform Trivia.
—The Fact Monster