The Collaborative International Dictionary of English v.0.48:

Slacken \Slack"en\, n. (Metal.)
   A spongy, semivitrifled substance which miners or smelters
   mix with the ores of metals to prevent their fusion. [Written
   also slakin.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Slack \Slack\, Slacken \Slack"en\, v. t.
   1. To render slack; to make less tense or firm; as, to slack
      a rope; to slacken a bandage. --Wycklif (Acts xxvii. 40)
      [1913 Webster]

   2. To neglect; to be remiss in. [Obs.] --Shak.
      [1913 Webster]

            Slack not the pressage.               --Dryden.
      [1913 Webster]

   3. To deprive of cohesion by combining chemically with water;
      to slake; as, to slack lime.
      [1913 Webster]

   4. To cause to become less eager; to repress; to make slow or
      less rapid; to retard; as, to slacken pursuit; to slacken
      industry. "Rancor for to slack." --Chaucer.
      [1913 Webster]

            I should be grieved, young prince, to think my
            presence
            Unbent your thoughts, and slackened 'em to arms.
                                                  --Addison.
      [1913 Webster]

            In this business of growing rich, poor men should
            slack their pace.                     --South.
      [1913 Webster]

            With such delay
            Well plased, they slack their course. --Milton.
      [1913 Webster]

   5. To cause to become less intense; to mitigate; to abate; to
      ease.
      [1913 Webster]

            To respite, or deceive, or slack thy pain
            Of this ill mansion.                  --Milton.
      [1913 Webster]

   Air-slacked lime, lime slacked by exposure to the air, in
      consequence of the absorption of carton dioxide and water,
      by which it is converted into carbonate of lime and
      hydrate of lime.
      [1913 Webster]

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.
      [1913 Webster]

   2. To be remiss or backward; to be negligent.
      [1913 Webster]

   3. To lose cohesion or solidity by a chemical combination
      with water; to slake; as, lime slacks.
      [1913 Webster]

   4. To abate; to become less violent.
      [1913 Webster]

            Whence these raging fires
            Will slacken, if his breath stir not their flames.
                                                  --Milton.
      [1913 Webster]

   5. To lose rapidity; to become more slow; as, a current of
      water slackens.
      [1913 Webster]

   6. To languish; to fail; to flag.
      [1913 Webster]

   7. To end; to cease; to desist; to slake. [Obs.]
      [1913 Webster]

            That through your death your lineage should slack.
                                                  --Chaucer.
      [1913 Webster]

            They will not of that firste purpose slack.
                                                  --Chaucer.
      [1913 Webster] Slack

WordNet (r) 3.0 (2006):

slacken
    v 1: become slow or slower; "Production slowed" [syn: slow,
         slow down, slow up, slack, slacken]
    2: make less active or fast; "He slackened his pace as he got
       tired"; "Don't relax your efforts now" [syn: slack,
       slacken, slack up, relax]
    3: become looser or slack; "the rope slackened"
    4: make slack as by lessening tension or firmness [syn:
       slacken, remit]

Moby Thesaurus II by Grady Ward, 1.0:

180 Moby Thesaurus words for "slacken":
   abate, allay, alleviate, anesthetize, appease, arrest, assuage,
   attemper, attenuate, backpedal, backwater, bank the fire, bate,
   benumb, block, blunt, bog down, bottle up, brake, chasten, check,
   clip the wings, come apart, confine, constrain, control,
   countercheck, curb, cushion, dam up, damp, dampen, de-emphasize,
   deaden, deaden the pain, decelerate, degenerate, delay, detain,
   deteriorate, die down, diffuse, dilute, diminish, disimprove,
   disintegrate, disjoin, disperse, dissolve into chaos, downplay,
   drag, draw rein, dull, ease, ease matters, ease off, ease up, ebb,
   extenuate, fall, fall back, foment, free, get worse, give relief,
   grow worse, hang up, hinder, hold back, hold in check, hold up,
   impede, inhibit, intercept, interfere, intermeddle, interrupt,
   intervene, keep back, keep in check, keep within bounds, lag, lax,
   lay, lenify, lessen, let down, let go, let up, let up on, lighten,
   loose, loosen, lose ground, lose momentum, lose speed, lull,
   make late, meddle, mire, mitigate, moderate, modulate, mollify,
   numb, obstruct, obtund, oppose, pad, palliate, play down, poultice,
   pour balm into, pour oil on, reduce, reduce the temperature, reef,
   regress, rein in, relapse, relax, relent, relieve, remit, repress,
   resist, restrain, retard, retrograde, retrogress, salve, scatter,
   scotch, set back, sicken, slack, slack off, slack up, slackening,
   slake, slip back, slow, slow down, slow up, slow-up, smother, snub,
   sober, sober down, soften, soothe, stay, stifle, stop, stupe,
   subdue, subside, suppress, take in sail, tame, temper,
   throttle down, tone down, tune down, unbend, unbrace, underplay,
   unglue, unlax, unleash, unstick, unstrain, unstring, untighten,
   untune, unwind, wane, water down, weaken, worsen