A shortened version of "Undincamisile", a Xhosa term to express shock. Undincamisile can also mean "you have kissed me, you kissed me, s/he kissed me", which is not related to the definition we are talking about here.
Girl 1: She has already moved on.
Girl 2: For real? It's only been 2 weeks.
Girl 1: Dead serious.
Girl 2: Ncmsl