Because BMW exclusively produced motorbikes at the time, the term “beamer” was created to describe one of those vehicles. The majority of people are unaware that the term “beamer”—which should properly be “bimmer”—is wrong for the vehicle.
BMW began making motorcycles. Their popularity on the racetrack became BSA Bikes BMW’s main rival. Because the initials were so close together, they became difficult to pronounce, and track fans came up with a slang term for them.
While BSAs were referred to as beesers, BMWs were called beamers. When the first BMW vehicles were released, those same fans decided that the car couldn’t have the same lingo as the motorcycle, so bimmer rather than beamer was the correct slang to help distinguish the car and the motorcycle. However, over time, beamer came to be used to describe both.