The Collaborative International Dictionary of English v.0.48:

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.
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   2. Weak; not holding fast; as, a slack hand. --Milton.
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   3. Remiss; backward; not using due diligence or care; not
      earnest or eager; as, slack in duty or service.
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            The Lord is not slack concerning his promise, as
            some men count slackness.             --2 Pet. iii.
                                                  9.
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   4. Not violent, rapid, or pressing; slow; moderate; easy; as,
      business is slack. "With slack pace." --Chaucer.
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            C?sar . . . about sunset, hoisting sail with a slack
            southwest, at midnight was becalmed.  --Milton.
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   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.
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   Syn: Loose; relaxed; weak; remiss; backward; abated;
        diminished; inactive; slow; tardy; dull.
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WordNet (r) 3.0 (2006):

slack water
    n 1: a stretch of water without current or movement; "suddenly
         they were in a slack and the water was motionless" [syn:
         slack, slack water]
    2: the occurrence of relatively still water at the turn of the
       (low) tide [syn: slack water, slack tide]