When you tell someone that he’s toast, what you mean is that he’s in big trouble.
Examples: When Jai’s mother found out that he had broken her favourite flower vase, he was toast. ”When the Principal walked into the classroom looking really angry, I knew we were toast.”
When somebody says, `I failed to complete the project’, you may respond by saying, `You’re toast, my friend’.
The expression is used in informal contexts.