Slang, for "meu irmão", which would translate to "my bro". You can use with other lads. Women rarely use, as it would sound as they were trying to portray a man, or being sarcastic, etc. Never use it in formal situations.
- Coé mermão! Como você entra sem bater na porta?
- Opa mermão! Como vai a vida mansa?
- Ah, sempre de boa né?