expression, suru verb (irregular)
take seriously; believe; fall for
Used when someone treats a statement, joke, or situation as genuine or important, often in negative or cautionary contexts.
彼の冗談を本気にしないでください。
Please don't take his joke seriously.
そんな話を本気
Nobody believes a story like that.
彼女は彼の甘い言葉を本気にしてしまった。
She fell for his sweet talk.