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)
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.)
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)