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)
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
Unbent your thoughts, and slackened 'em to arms.
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
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.