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late Old English sleacian, slacian "become slack or remiss; slacken an effort" (intransitive); "delay, retard" (transitive), from slæc "lax" (see slack (adj.)). Transitive sense of "make slack" is from late 12c. Sense of "allay, diminish in force, quench, extinguish" (in reference to thirst, hunger, desire, wrath, etc.) first recorded early 14c. via notion of "make slack or inactive." Related: Slaked; slaking.