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.

Source : WordNet®

slack water
     n : the occurrence of relatively still water at the turn of the
         (low) tide [syn: {slack tide}]