'Deadass' is American slang, especially from New York, meaning 'seriously' or 'I'm not joking'. 'Deadass, I saw it' = 'I swear, I saw it'. The 'dead' part intensifies the seriousness, not any morbid meaning. If your teen sprinkles 'deadass' through messages they are emphasising honesty. Harmless, conversational, very common in Gen Z American speech and spreading globally through hip-hop and TikTok.