In this video, the older guy (Brazilian) calls the younger guy (his nephew) something that sounds like “Ma-MAU” or “Mo-MAU.” That’s not the younger man’s name - I know this because he addresses another younger man (another nephew) the same way in other videos.
Almost certainly meu irmão, meaning “my brother”, kind of like saying “my man”…
It is often spoken quickly without full enunciation.
ETA: I didn’t realize you had a video…I looked at it and stand by my statement, though the accent is from somewhere other than Rio. It’s the kind of thing guys say to each other all the time.
I had a Puerto Rican coworker that used it. One particular way was in playful teasing after someone made a boneheaded but harmless mistake. Sort of like one might say ‘Nice grip there, butterfingers’ if I dropped a screwdriver. But it was also funny used as a general address like ‘What time you mamaus taking lunch?’
He doesn’t work here anymore but we still sometimes use the term.