in earnest

With purposeful or sincere intent, as in We settled down to study in earnest. [c. a.d. 1000]

Also, in dead earnest. Serious, determined, as in We thought he was joking, but he was in earnest, or I’m in dead earnest about selling the business. In the variant, from the late 1800s, dead means “completely” or “thoroughly” and is used purely for emphasis. [c. a.d. 1000]