Etymology
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beat (n.)

c. 1300, "a beating, whipping; the beating of a drum," from beat (v.). As "throb of the heart" from 1755. Meaning "regular route travelled by someone" is attested from 1731, also "a track made by animals" (1736), from the sense of the "beat" of the feet on the ground (late Old English), or perhaps that in beat the bushes to flush game (c. 1400), or beat the bounds (1560s). Extended to journalism by 1875. Musical sense is by 1842, perhaps from the hand motion of the conductor and the notion of "beating the time":

It is usual, in beating the time of a piece of music, to mark or signalize the commencement of every measure by a downward movement or beat of the hand, or of any other article that may be used for the purpose .... ["Godfrey Weber's General Music Teacher," 1842]

Earlier in music it meant a sort of grace note:

BEAT, in music, a transient grace note, struck immediately before the note it is intended to ornament. The beat always lies half a note beneath its principal, and should be heard so closely upon it, that they may almost seem to be struck together. ["The British Encyclopedia," London, 1809]
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