also: マジメくさる
godan verb (-ru), intransitive verb
put on a serious face; act overly solemn; become dead serious
Often used critically or humorously when someone suddenly adopts an exaggeratedly serious attitude. Frequently appears in the forms 真面目くさった or 真面目くさって.
急に真面目くさった顔で話し始めた。
He suddenly started talking with a dead serious face.
そんなに真面目くさらなくてもいいよ。
You don't have to be so overly serious.