郑重其事；严肃认真
completely serious
These fanatics are in deadly earnest when they say they want to destroy all forms of government. 这些狂热分子声称要摧毁所有形式的政府时，他们可不是说着玩，是完全当真的。
"Surely she was joking?" "Absolutely not. She was in deadly earnest."
His expression changed and I realised he was in deadly earnest.
She smiled but I knew she was in deadly earnest.
He told me, in deadly earnest, that he knew he was the man for the job.
See also: earnest