Source : Webster's Revised Unabridged Dictionary (1913)

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.