Example 1:
Mike: Hey Jennifer, how's it going?
Jennifer: Oh Mike, pretty good--but I didn't get any sleep last night... my neighbors were partying late AGAIN!
Mike: I see... Did you tell them to quiet down? Also, maybe you should tell them that if they do it again, you'll call the police.
Jennifer: Actually, I did! But they don't seem to take me seriously...
Mike: Hey Jennifer, how's it going?
Jennifer: Oh Mike, pretty good--but I didn't get any sleep last night... my neighbors were partying late AGAIN!
Mike: I see... Did you tell them to quiet down? Also, maybe you should tell them that if they do it again, you'll call the police.
Jennifer: Actually, I did! But they don't seem to take me seriously...
Mike: Well, I think you should tell them that you're dead serious about calling the police next time.
Jennifer: Good advice, Mike. I really am dead serious about this. I need my sleep!
Example 2:
Tony: Lisa, I've been thinking maybe we shouldn't live together anymore, but I would still like for us to date each other...
Lisa: What? Listen, Tony, if we stop living together--then we should just break up.
Tony: But I really like you! Are you serious?
Lisa: I'm dead serious! We've already been seeing each other for two years and I'm ready for the next step. I need a commitment.
Meaning: absolutely serious, not joking
The idiom "dead serious" was taken from Unit 5 ("Hanging Out") in LSI's textbook Speaking Savvy for Level 5 Speaking/Listening classes.