Same as no. Has many other uses and is more flexible. Can be used to shut someone up mid sentence
Example of No:
"Can I have some ?" "NO DEAL!"
Example of Flexibility:
"I love you" "NO DEAL!"
Another term for "no", or "go fuck yourself". When someone says something to you, usually presenting an offer or an ultimatum, you can disagree by saying "no deal"
Hey man I just found out that you've been flirting with my girlfriend and now I'm very angry. What am I going to do about this you may ask? Well, I'm gonna come to your house, kick your teeth in, and shut your mouth for you.