noun, noun or participle which takes 'suru', intransitive verb
smiling broadly; breaking into a smile
A yojijukugo describing a sudden, broad smile that lights up the face, often after tension or seriousness. Used as a noun or with する as an intransitive verb.
彼の冗談に、彼女は破顔一笑した。
At his joke, she broke into a broad smile.
厳しい表情だった祖父が、孫の顔を見て破顔一笑した。
My grandfather, who had a stern expression, broke into a smile when he saw his grandchild.