in earnest

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]

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