You were having a friendly disagreement with your friend about who's going to win a baseball game. You said that you'd bet $100 that the Mets would lose. Your friend asked if you were serious about the bet. You are, so you answer:
Yeah, I'm dead serious.
"Yeah" means "yes" but in a more casual way.
"Dead serious" means "completely serious". You use this phrase to express being very serious about something you said. It's a good response when someone says they don't believe you:
A: No way, man!
B: No, I'm dead serious!
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