Another word used to describe a condom.
To use a condom, to cover one's penis for protection, hence its association with a raincoat. Generally made of water repellent material, a raincoat is similar to the make up of condoms.
More often in reference to seeking protection from STDs than for protection against pregnancy.
Noteworthy: Mainly a high school term, where one rarely used the common term of prophylactic. An older generation thing.