на полном серьёзе(Mary's comments were in earnest. She really meant them. • We thought he was joking at first, but then we realized that he was in earnest.Anglophile)
не на шутку(разыграться и т.п. • Closer to midnight, the rain began in earnest. • After light showers during the day, it began to rain in earnest in the evening. • This time Palestinians and Israelis began killing each other in earnest, as their conflict spun out of control. • With the collapse of a 2-year ceasefire, Turkish forces and Kurdish militants are again killing each other in earnest.Anglophile)
всерьёз(We thought he was joking at first, but then we realized that he was in earnest. • She started studying snakes in earnest. • The deposit was discovered in the late 1980s, and planning for a mine began in earnest about 15 years ago.)