What does 非也 mean?
I heard the following:
Is this XXX? 非也! It's YYY!
Does it mean "Oh my god!" or something others?
也 is classified into 虚词(lit. imaginary word) in classical Chinese. 虚词, unlike its counterpart of 实词(lit. real word), doesn't have a meaning, but it's indispensable to some grammatical functions. It can:
也 is generally used at the end of a sentence to complete a declarative sentence, so in your case, 非也 means "(it's) not (so)".
generally you won't use this pattern today, but you can use it in a funny way. As in English, you can say "thy bidding, master" :)
非也 literally means "not also". There's an implication that you've given more than one wrong answer in the recent past. OR literally "you should already know the answer to this question". Or in hip-hop slang, "you ain't know?"
非 - means 'not' or 'without'. 也 - isn't just simply a modal, it means "also" which literally implies a slight 'annoyance' with your question and while it is not generally used in big cities, it is still very much used in villages and the countryside (as I've watched it used on several current Chinese TV shows that center around village life within the past few years).
Sometimes Chinese slang really reminds me of so-called Ebonics in the west. There is a 'feigned annoyance' with your mates or buddies that is actually a tool for bonding.