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1. Lit. [for a horse] to lean back on its hind legs and raise its front legs, assuming a threatening posture or avoiding something on the ground such as a snake. (See also rear back.) The horse reared up suddenly, throwing the rider onto the ground. When the horse reared up, I almost fell off.
2. Fig. [for something, especially a problem] to raise up suddenly. A new problem reared up and cost us a lot of time. A lot of new costs reared up toward the end of the month.
1. To rise on the hind legs, as of a horse: A rattlesnake slithered out from behind the bush, and the horse reared up.
2. To arise or appear suddenly or unexpectedly: We can handle any problems that rear up.