NTY means "No, Thank You!" It is a good and polite manner to simply deny an offer given by the other person for anything mean to you. It does not underestimate someone, just decline their offer because it is not acceptable at that moment. Let's check out the given example below.
Person A: The weather is very good outside, let's have a walk together. Are you up?
Person B: NTY, I'm not good yet. Schedule it tomorrow.

Here is the conversation between two people giving us an overview regarding the simple "no" that might cost you more than simply saying this slang to give them a perfect opinion to refuse the offer.