To suddenly begin to cry; to burst into tears or start crying (often from emotion or pain).
赤ちゃんが急に泣き出したので、母親が抱き上げた。
The baby suddenly began crying, so the mother picked him up.
映画の最後の場面で彼女はこらえきれずに泣き出した。
She couldn't hold back and burst into tears at the film's final scene.
試験に失敗して、彼は学校の廊下で泣き出した。
After failing the exam, he began crying in the school hallway.
痛みで泣き出す子どもを見て、先生はすぐ手当てをした。
Seeing the child cry out in pain, the teacher immediately gave first aid.