The Collaborative International Dictionary of English v.0.48:
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
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.
5. To lose rapidity; to become more slow; as, a current of
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.
They will not of that firste purpose slack.
[1913 Webster] Slack