expression, ichidan verb
leave to chance; let things take their course; go with the flow
Used when one deliberately refrains from controlling a situation and instead lets events unfold naturally. Often implies a relaxed or resigned attitude.
結果は成り行きに任せよう。
Let's leave the outcome to chance.
彼は計画を立てずに、いつも成り行
He never makes plans; he always lets things take their course.