Wiktionary
n. The sound produced by a person uttering an evil laugh, frequently represented as "muahahaha."
Wikipedia
Evil laughter or maniacal laughter is a stock manic laughter by a villain in fiction. The expression "evil laugh" dates back to at least 1860. "Wicked laugh" can be found even earlier, dating back to at least 1784. Another variant, the "sardonic laugh," shows up in 1714 and might date back even further.
In comic books, where supervillains utter such laughs, it is variously rendered as mwahahaha, mwahaha, muwhahaha, muahahaha, buahahaha, bwuhuhuhaha, etc. (Compare with Ho ho ho.) These words are also commonly used on internet blogs, bulletin board systems, 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 interjections, and less frequently as nouns.
During the 1930s, the popular radio program The Shadow used a signature evil laugh as part of its presentation. This was voiced by actor Frank Readick, and his laugh was used even after Orson Welles took over the lead role. The most recognizable and copied evil laugh is probably the one voiced by Vincent Price, as it has been used or copied in radio, film, music, and television, notably at the end of the music video Michael Jackson's Thriller.
In films, evil laughter often fills the soundtrack when the villain is off-camera. In such cases, the laughter follows the hero or victim as they try to escape. An example is in Raiders of the Lost Ark, where Belloq's laugh fills the South American jungle while Indiana Jones attempts to escape from the Hovitos.
Non-human characters such as King Ghidorah and Destoroyah from the Godzilla series can also have extremely unique and sinister laughs or laughter-like sounds.
Usage examples of "evil laughter".
From some distance came a burst of evil laughter and the word Silence .
The giant was indeed closing for the kill, lifting its deadly weapon, when an arrow thudded hard into its back, turning its evil laughter into a sudden wheeze.
Into the night there came a cry of pure terror, evil laughter, a gloating triumph.
The noise should have been reassuring, but it sounded mocking, as if the labyrinth had taken the joyful notes and twisted them into evil laughter.
It was sobering and terrible, the vampire elated, his evil laughter a harsh echo in Paul's mind.
She heard laughter, evil laughter which scorched her, though her body shivered as the last of her clothing was ripped away.
The evil laughter had died away along with the clangor of drums, bells, and gongs.