The Collaborative International Dictionary of English v.0.48:

Slack \Slack\, Slacken \Slack"en\, v. i. [imp. & p. p.
   Slacked, Slackened; p. pr. & vb. n. Slacking,
   Slackening.] [See Slack, a.]
   1. To become slack; to be made less tense, firm, or rigid; to
      decrease in tension; as, a wet cord slackens in dry
      weather.
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   2. To be remiss or backward; to be negligent.
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   3. To lose cohesion or solidity by a chemical combination
      with water; to slake; as, lime slacks.
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   4. To abate; to become less violent.
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            Whence these raging fires
            Will slacken, if his breath stir not their flames.
                                                  --Milton.
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   5. To lose rapidity; to become more slow; as, a current of
      water slackens.
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   6. To languish; to fail; to flag.
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   7. To end; to cease; to desist; to slake. [Obs.]
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            That through your death your lineage should slack.
                                                  --Chaucer.
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            They will not of that firste purpose slack.
                                                  --Chaucer.
      [1913 Webster] Slack

WordNet (r) 3.0 (2006):

slackening
    n 1: an occurrence of control or strength weakening; "the
         relaxation of requirements"; "the loosening of his grip";
         "the slackening of the wind" [syn: relaxation,
         loosening, slackening]

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "slackening":
   abatement, allayment, alleviation, arrest, assuagement,
   attenuation, attrition, blunting, calming, check, damping,
   deadening, debilitation, deceleration, delay, demulsion, detention,
   devitalization, dilution, diminution, drag, dulcification, dulling,
   ease-off, ease-up, easing, effemination, enervation, enfeeblement,
   evisceration, exhaustion, extenuation, falling-off, fatigue,
   flagging, holdup, hushing, inanition, lag, languishment, leniency,
   lessening, letdown, letup, lightening, loosening, lulling,
   minus acceleration, mitigation, modulation, mollification,
   obstruction, pacification, palliation, quietening, quieting,
   reduction, relaxation, remission, retardation, retardment, setback,
   slack-up, slowdown, slowing, slowing down, slowup, softening,
   soothing, subduement, tempering, thinning, tranquilization,
   weakening