dead serious

Translations
совершенно серьезный
She was dead serious about quitting her job.
Она была совершенно серьезна в своем намерении уволиться с работы.
Definitions
A phrase used to indicate that someone is not joking and is completely sincere or earnest about what they are saying.
Words
dead

adverb
совершенно
serious

adjective
серьёзный