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1. To lean or totter and fall over; to collapse. The strong winds caused a number of buildings in the city to topple over. Someone knocked into the old lady and she toppled over on the sidewalk.
2. To cause someone or something to lean or totter and fall over; to cause someone or something to collapse. In this usage, a noun or pronoun can be used between "topple" and "over." The child toppled the blocks over with a kick. The performer lost control of his unicycle and toppled over a bunch of people in the audience.
[for something very tall] to fall over. I was afraid that Jimmy's stack of blocks would topple over. The stack of books toppled over and ended up as a jumbled mess on the floor.