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".