To suddenly cry out or shout; to break into a cry (often from surprise, pain, or strong emotion).
映画の怖い場面で隣の人が急に叫びだした。
At the scary scene in the movie, the person next to me suddenly cried out.
痛みで彼は思わず叫びだした。
He involuntarily cried out from the pain.
ゴールが決まった瞬間、スタジアム中が歓声を叫びだした。
The moment the goal was scored, the whole stadium erupted in cheers.