I probably should have added more info when I first posted. This is a page from a 1750 stato dell'anime. The line reads:
"Angela Lisabetta Maria Troisi, sua nezza, e figlia di Filippo e q. Petronilla di Garofano..... yada yada .... age 11."
The woman on the (cut-off) line above her was her grandmother, aged sixty, and according to the formula of the other households, "sua nezza" describes her relationship to the head of the house (in this case her grandmother.)
In the previous censuses "nipote" was used for niece/nephew/grandchild, so this "nezza" thing is a new twist.
I've scoured the Latin books and can't find anything remotely similar and I know it's not dialect. It's bugging me that I can't find a definition.....