Brewer's Dictionary of Phrase and Fable/Abate
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Abate (2 syl.) means properly to knock down. (French, abattre, whence a battue, i.e., wholesale destruction of game; O.E. a beátan.)
Abate, in horsemanship, is to perform well the downward motion. A horse is said to abate when, working upon curvets, he puts or beats down both his hind legs to the ground at once, and keeps exact time.