c. 1300, "to deal a blow;" later "to strike with a whip, beat with a lash" (late 14c.), possibly imitative. To lash out "to strike out violently" (originally of horses) is from 1560s and preserves the older sense. Related: Lashed; lashing.
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Definitions of lashing from WordNet
beating with a whip or strap or rope as a form of punishment;