訓 つ(く) ・ 音 トツ,カ
Sino-Vietnamese: đột
Hole 穴 over 大 (originally 犬, dog): a dog suddenly bursting out of its den, so it means thrust or sudden (Han Viet dot, as in dot nhien = suddenly).
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突然、雨が降り出しました。 (とつぜんあめがふりだしました)
Suddenly it started to rain.