“Yeah, I'm dead serious.”

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 this is your answer.

Yeah, I'm dead serious.

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Yeah

"Yeah" is a more casual way of saying "yes". 

Say this to agree with something that a person said:

Yeah, no problem.

Yeah, I actually did it myself.

You can also say "yeah" when you're going to disagree but you want to make your disagreement sound a little softer.

Yeah, but then it'll be blocking one of the outlets.

"Yeah" sounds less formal than "yes." 

(someone) is dead serious

"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!