This is a most useful word, meaning ignorant. It’s unknown to most people, with which you may bait your opponents: if they don’t know the word, then their ignorance is doubly obvious.
It’s rare, though it appears in Ulysses by James Joyce, in which he speaks of “the lethargy of nescient matter”. The noun, nescience, is somewhat more common; G K Chesterton used it in The Innocence of Father Brown: “Flambeau had been missed at Harwich; and if he was in London at all, he might be anything from a tall tramp on Wimbledon Common to a tall toast-master at the Hotel Metropole. In such a naked state of nescience, Valentin had a view and a method of his own”.
Nescient comes from Latin nescire, to be ignorant, from scire, to know. This is the same Latin stem that bequeathed us nice, a word which has gone through more shifts of sense than almost any other, but which started out meaning “foolish” or “stupid”.