Deadass is an informal intensifier associated with New York City speech. It can stress that someone is serious or ask whether a claim is true.[1]
Statement and question
“I deadass left my keys in the Uber” delivers bad news. “Deadass?” opens a brief fact-checking period before everybody has to go back outside.
Emphasis
Placed before a verb, it can stress what the speaker did. Used alone with rising intonation, it can express surprise or ask the other person to confirm what they just said.