Source : Webster's Revised Unabridged Dictionary (1913)

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)

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

            Slack not the pressage.               --Dryden.

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

   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.

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

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

            With such delay Well plased, they slack their
            course.                               --Milton.

   5. To cause to become less intense; to mitigate; to abate; to
      ease.

            To respite, or deceive, or slack thy pain Of this
            ill mansion.                          --Milton.

   {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.

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.

   2. To be remiss or backward; to be negligent.

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

   4. To abate; to become less violent.

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

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

   6. To languish; to fail; to flag.

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

            That through your death your lineage should slack.
                                                  --Chaucer.

            They will not of that firste purpose slack.
                                                  --Chaucer.

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}.]

Source : WordNet®

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