You need to understand the phrase as "I'm deadass (serious)." It works like this: I'm serious. I'm dead serious. (This is standard English. Here, "dead" means "absolutely".) I'm deadass serious. (We add the vulgar "ass" for emphasis.) To be honest, I wouldn't ever use this expression, but that's because of the region I come from. It's better to find out which native English speakers actually use this expression.

US here - That's not a common expression. It wouldn't sound native. (People would probably assume you're translating something from your native language that doesn't quite work in English.)

We use it all the time where I'm from. I would type more but I'm dead ass tired..... ;)