"I'm going over her house after school"
"Yea, deadass, I'm going there."
"She hit her right in her face!"
"No she didn't. I don't believe you!"
"Deadass, she just reached over and popped her! pow pow!"
'Deadass' means 'I swear'. It is used to emphasize how much you mean something that someone may doubt. It means 'For Real, For Real"
"My dog ate my homework."
"I'm deadass, he gobbled it right up!"
After years of hearing it, until today, I didn't really even think of it as a unique form of expression. For whatever reason, when today a student promised me that she was staying for after school tutoring, it struck me how strange the phrase was.
Student A: "I'm staying today."
Me: "Oh good, I'm so glad."
Student B: "Nothing's for sure."
Student A: "No, I'm deadass, I'm staying."
Me: "Oh well if you're dead-a, that means you're serious."
Student A: "Yea" (with no hint of amusement at her own way of expressing her seriousness).