It doesn't happen often. In fact, it's estimated that fewer than 5 percent of U.S. brand names become generic. Those that do typically are inventions or products that improve on what's already on the market. The brand names then become so popular that they eclipse rivals in sales, market share and in the minds' of consumers. And then they spread through the English language like the common cold in a small office.
A company's biggest fear is that their brand name becomes so commonly used to describe a product that a judge rules that it's too "generic" to be a trademark. That means that any product — even inferior ones — can legally use the name. A brand usually is declared legally generic after a company sues another firm for using its name and the case goes to a federal court.
Drug maker Bayer lost trademarks for the names "aspirin" and "heroin" this way in the 1920s. So did B.F. Goodrich, which sued to protect its trademark of "zipper" in the 1920s after the name joined the world of common nouns. Similar cases deemed "escalator" generic in 1950, "thermos" generic in 1963 and "yo-yo" generic in 1965.
It's difficult to quantify how much revenue a company loses when its brand is deemed generic. But companies worry that it breeds confusion among consumers.