A Prairie-Masker is when a girl gives a blowjob while she has an Alka-seltzer tablet and some water in her mouth.
The definition was coined abord U.S Naval submarines, named after the Prairie-Masker anti-sonar detection device employed by submarines. The Prairie-Masker anti-sonar system works, in a nutshell, by creating a barrier of bubbles around the ship thus reducing noise and detection by hostile sonar devices.
Jake: "Hey Greg, how was shore leave?"
Greg: "Not bad, I got a local girl to give me a Prairie-masker."