Definition: Bimethyl (also known as "methyl") is a slang term used to describe a person who seems like they're having fun, but really just doing something mean and annoying. Imagine someone wearing sunglasses that make them look cool, but when they talk or act unpleasantly, the glasses are their mask. It's not about being nice, it's more about being smart and funny.

An example sentence: "She was bimethyl for two hours straight, never giving me a second thought."