Onomatopoeia of laughter. As stated specifically by Curly of the Three Stooges. Usually written and spoken sucessively.
"Oh, a wise guy, eh? nyuk, nyuk, nyuk."
From the Three Stooges, Curly's laugh.
Moe: Why I outta...
*Sound of Larry's head getting slapped*
Curly: nyuk nyuk nyuk...
Moe: Shuttup you
*Sound of Curly's eyes getting poked*