My guess is that "nam saeng" is just a name. For example, a figure in the old Koguryo dynasty has the name 연남생 (Yeon Nam Saeng). It might be that there is a period drama covering this era (7th century) and you picked up this name from it or some article talking about it.

I looked up the word in the dictionary, but I can't find the word. Maybe it's a word someone made by combining Chinese characters. In this case, the meaning of the word varies and you will have to check the context and deduce the meaning.