So you know how we always thought Jason was saying, “Ch ch ch. Ha ha ha.” Well, it turns out we were all wrong.

“The spelling of those two syllables has become a hotly contested issue among diehard films of the franchise. Most people have been repeating it as ch ch ch, ah ah ah for years, and it’s hard to hear it any other way. However, the film’s composer, Harry Manfredini, has set the record straight in past interviews. In 2015, he told Gun Media that the sounds are actually ki and ma.

Inspired by the consonant-heavy Polish scores he was studying at the time, Manfredini decided to reduce the words kill and mommy down to two syllables. These words were taken directly from a scene in the original 1980 film in which Pamela Voorhees, played by Betsy Palmer, says “Kill her, mommy!” in Jason’s voice. Manfredini said the scene stuck out as a “creepy shot” and also conveyed the important idea that she was hearing voices at all times.”

Source: Mental Floss – The Real Story Behind Friday the 13th‘s Iconic Whisper Sound Effect