The Collaborative International Dictionary of English v.0.48:

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)
      [1913 Webster]

   2. To neglect; to be remiss in. [Obs.] --Shak.
      [1913 Webster]

            Slack not the pressage.               --Dryden.
      [1913 Webster]

   3. To deprive of cohesion by combining chemically with water;
      to slake; as, to slack lime.
      [1913 Webster]

   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.
      [1913 Webster]

            I should be grieved, young prince, to think my
            presence
            Unbent your thoughts, and slackened 'em to arms.
                                                  --Addison.
      [1913 Webster]

            In this business of growing rich, poor men should
            slack their pace.                     --South.
      [1913 Webster]

            With such delay
            Well plased, they slack their course. --Milton.
      [1913 Webster]

   5. To cause to become less intense; to mitigate; to abate; to
      ease.
      [1913 Webster]

            To respite, or deceive, or slack thy pain
            Of this ill mansion.                  --Milton.
      [1913 Webster]

   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.
      [1913 Webster]