I am ashamed about this question, I did seek for its meaning, but nothing to help.
I am checking a method of calculation of some sheet from a Technical Manual of engineering, and, I find this: "n.g.", contrary to "OK" for results of calculus.
Please help me, about its meaning; I'm sorry for my ignorance.
It seems that acronym is referring to the fact that the tension in the material is greater than the strength thus the acronym could be “(safely) not guaranteed” or something similar as observed in the comments.