Source : Webster's Revised Unabridged Dictionary (1913)

Slack \Slack\, a. [Compar. {Slacker}; superl. {Slackest}.] [OE.
   slak, AS. sleac; akin to OS. slak, OHG. slah, Prov. G.
   schlack, Icel. slakr, Sw. slak; cf. Skr. s[.r]j to let loose,
   to throw. Cf. {Slake}.]
   Lax; not tense; not hard drawn; not firmly extended; as, a
   slack rope.

   2. Weak; not holding fast; as, a slack hand. --Milton.

   3. Remiss; backward; not using due diligence or care; not
      earnest or eager; as, slack in duty or service.

            The Lord is not slack concerning his promise, as
            some men count slackness.             --2 Pet. iii.
                                                  9.

   4. Not violent, rapid, or pressing; slow; moderate; easy; as,
      business is slack. ``With slack pace.'' --Chaucer.

            C?sar . . . about sunset, hoisting sail with a slack
            southwest, at midnight was becalmed.  --Milton.

   {Slack in stays} (Naut.), slow in going about, as a ship.

   {Slack water}, the time when the tide runs slowly, or the
      water is at rest; or the interval between the flux and
      reflux of the tide.

   {Slack-water navigation}, navigation in a stream the depth of
      which has been increased, and the current diminished, by a
      dam or dams.

   Syn: Loose; relaxed; weak; remiss; backward; abated;
        diminished; inactive; slow; tardy; dull.

Slack \Slack\, n. [Cf. {Slag}.]
   Small coal; also, coal dust; culm. --Raymond.

Slack \Slack\, n. [Icel. slakki a slope on a mountain edge.]
   A valley, or small, shallow dell. [Prov. Eng.] --Grose.

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\, adv.
   Slackly; as, slack dried hops.

Slack \Slack\, n.
   The part of anything that hangs loose, having no strain upon
   it; as, the slack of a rope or of a sail.

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.

Source : WordNet®

slack
     adj 1: not tense or taut; "the old man's skin hung loose and gray";
            "slack and wrinkled skin"; "slack sails"; "a slack
            rope" [syn: {loose}]
     2: lacking in strength or firmness or resilience; "flaccid
        muscles"; "took his lax hand in hers"; "gave a limp
        handshake"; "a limp gesture as if waving away all desire
        to know" G.K.Chesterton; "a slack grip" [syn: {flaccid}, {lax},
         {limp}]
     3: flowing with little speed as e.g. at the turning of the
        tide; "slack water"
     4: lacking in rigor or strictness; "such lax and slipshod ways
        are no longer acceptable"; "lax in attending classes";
        "slack in maintaining discipline" [syn: {lax}]

slack
     v 1: avoid responsibilities and work, be idle
     2: be inattentive to, or neglect; "He slacks his attention"
     3: release tension on; "slack the rope"
     4: make less active or fast; "He slackened his pace as he got
        tired"; "Don't relax your efforts now" [syn: {slacken}, {slack
        up}, {relax}]
     5: become slow or slower; "Production slowed" [syn: {slow}, {slow
        down}, {slow up}, {slacken}]
     6: make less active or intense [syn: {slake}, {abate}]
     7: become less in amount or intensity; "The storm abated"; "The
        rain let up after a few hours" [syn: {abate}, {let up}, {slack
        off}, {die away}]
     8: cause to heat and crumble by treatment with water; "slack
        lime" [syn: {slake}]

slack
     n 1: dust consisting of a mixture of small coal fragments and
          coal dust and dirt that sifts out when coal is passed
          over a sieve
     2: a noticeable deterioration in performance or quality; "the
        team went into a slump"; "a gradual slack in output"; "a
        drop-off in attendance"; "a falloff in quality" [syn: {slump},
         {drop-off}, {falloff}, {falling off}]
     3: a stretch of water without current or movement; "suddenly
        they were in slack water"
     4: the condition of being loose (not taut); "he hadn't counted
        on the slackness of the rope" [syn: {slackness}]
     5: a cord or rope or cable that is hanging loosely; "he took up
        the slack"

Source : Free On-Line Dictionary of Computing

slack
     
        1.  Internal fragmentation.  Space allocated
        to a disk file but not actually used to store useful
        information.
     
        2.  In the theology of the {Church of the SubGenius},
        a mystical substance or quality that is the prerequisite of
        all human happiness.
     
        Since {Unix} files are stored compactly, except for the
        unavoidable wastage in the last block or fragment, it might be
        said that "Unix has no slack".
     
        See {ha ha only serious}.
     
        [{Jargon File}]
     
        (1995-03-01)