From The Art and Popular Culture Encyclopedia
An evil laugh is a stock manic laugh by a villain in fiction. In comic books, where supervillains utter such laughs, it is variously rendered as mwahahaha, muwhahaha, muahahaha, bwahahaha, pwahahaha, puahahaha or kyahahaha (female version) (Compared to Ho ho ho). These words are also commonly used on internet Blogs, BBSes, and games. There, they are generally used when some form of victory is attained, or to indicate superiority over someone else. The words are often used as either interjections or less frequently, as nouns. An evil laugh is often entertaining for friends because each one may be unique. The most recognizable and copied evil laugh is probably the one voiced by the late Vincent Price, as it has been used or copied in radio, film, music, and television.
In films, the evil laugh often fills the soundtrack even though the villain is off-camera. The laugh therefore follows the hero or victim as they try to escape. Examples include Raiders of the Lost Ark, where Belloq's laugh fills the South American jungle while Indiana Jones attempts to escape from the Hovitos, and Predator, where the Predator's evil laugh fills the jungle just before its self-destruct mechanism detonates.