- Nah means no.
- You can use it the same way you use no to respond to questions, but remember that it’s very casual. Using nah in formal situations may seem disrespectful.
Even though it has become common in informal communication facilitated by mobile phones and the Internet, nah has been in use since the first half of the twentieth century at least.
The Meaning of Nah
Nah is an informal version of the word no. It’s often used in text messages and casual online writing.
The first written use of nah can be dated back to 1920, when it likely came about as a part of an eye dialect—a nonstandard way of spelling words to represent the pronunciation they have in different dialects. Nah is a pronunciation found in some American dialects as well as in Cockney English.
How to Use Nah
Nah isn’t totally interchangeable with no. No can be an adverb or an adjective, but nah is an adverb only:
When using nah in informal communication like text messages, you should follow the capitalization rules you otherwise use for these communications. For example, it’s not uncommon to use lowercase letters in text messages even when you should capitalize. If you choose to use nah in formal communication, use the same rules you would for no.
from Grammarly Blog https://www.grammarly.com/blog/nah-meaning/