It's a mexican slang that has 3 meanings:
1. it's a vulgar or informal way to say "you're kidding"
2. it's a vulgar way to say "stop messing around"
3. sometimes can be used like saying "no way"
4. or when you're scared or desperate and you suddenly say "no mames" just because nervous or something like that and when you say that word is like saying "damn".
guy1: Karina said that you're going around with Jose
girl1: no mames !!!