Much like "anon", this term is used to refer to other anonymous 4chan users without just writing "you", but in a more feminine way since "nona" sounds feminine due to it ending in an "a".
It is primarily used by the denizens of 4chan's /lgbt/ board, where trans women make up a sizeable percentage of the posters.
Anonymous OP: What FFS procedures should I get so I can finally become a passoid rather than a gorilla ogrehon?
Anonymous responder: Nona, you already pass! Touch grass, you've gotten brainwormed and blackpilled.
When you are in a winning position in a time controlled game of chess, rather than moving or resigning your opponent just sits there and lets their clock tick down to 0.