It is uncertain where the word "존나" originated from. But lots of people think it's a contraction of "좆 나오게". "좆" is an informal word meaning a penis and "나오게" means come out. So, its literal meaning would be as much as a cock (dick) comes out. We use this expression to emphasize a adverb, adjective or verb like "very" in English. But again, it's nothing but a vulgar slang word which is prevailing among teens. It is not a right word for a decent girl like you to use and if you are a beginner of the Korean language, you would better turn off your interest in this kind of stuff and spend time to learn formal expressions.