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.