My son and I were discussing a legalism and he suggested that "ignorance of the law is no excuse" is not always correct usage. He contends that calling a person "ignorant" because he is unlearned, or uneducated about a matter depends upon whether he should have been aware of, or might have previously been oblivious to the matter. He recalled having learned another term, in a college English class, but which he could neither correctly pronounce, or spell. The best he could do was that it sounded like, "ness-e-ence." I couldn't find it in the dictionary either. Does anybody know this word and usage?
The word that your son was thinking of is nescience (and is pronounced as your son remembers).
nescience noun [ U ] /ˈnes.i.əns/ formal
the state of not knowing
"ignorance," 1610s, from Late Latin nescientia, from nesciens (see nescient).
Google Ngrams shows that by far the greatest usage of this word relates to Buddhist philosophy:
An extreme wing of Advaita holds the view that there is only one nescience and that nescience reflects Brahman and as soon as that reflected soul attains release there is the destruction of the nescience.
However, the usage that you posit is not really valid. "Ignorance of the law" is a fixed expression, and nescience does not appear to be a legal term.