A colloquial verb meaning to begin doing something (often suddenly or newly). It combines 'yaru' (do) + 'dasu' (to start), emphasizing the start of an action.
彼は急に部屋の掃除をやりだした。
He suddenly started cleaning the room.
授業が終わると、子どもたちは外で遊びをやりだす。
When classes finish, the children start playing outside.
彼女はプロジェクトの問題点を見つけると、一人で作業をやりだした。
When she found problems in the project, she began working on it by herself.