Kelly: I have some very big news!
Kelly: Yeah... I'm engaged!!!!
Joe: You're joking. I don't believe it.
Kelly: No, I'm dead serious. My boyfriend proposed to me last night. Look at my ring!
Joe: Wow! Congratulations!
When Tom told me that he got fired, I didn't really believe him. I was waiting for him to say he was just joking. Then he told me that he was dead serious. I feel really bad for him.
Meaning: to be dead serious is an idiom that is very common in American English. It means that you are not joking or kidding. It means that you are very serious and what you are saying is really true.
This idiom was taken from LSI's text book titled Speaking Savvy. This book is used to teach Speaking and Listening in our Level 5 class. For more information please visit: http://www.languagesystems.com/