Noah Webster's New International Dictionary of the English Language

1. (n.) Small coal; also, coal dust; culm.

2. (n.) A valley, or small, shallow dell.

3. (superl.) Lax; not tense; not hard drawn; not firmly extended; as, a slack rope.

4. (superl.) Weak; not holding fast; as, a slack hand.

5. (superl.) Remiss; backward; not using due diligence or care; not earnest or eager; as, slack in duty or service.

6. (superl.) Not violent, rapid, or pressing; slow; moderate; easy; as, business is slack.

7. (adv.) Slackly; as, slack dried hops.

8. (n.) The part of anything that hangs loose, having no strain upon it; as, the slack of a rope or of a sail.

9. (a.) Alt. of Slacken

10. (v. t.) Alt. of Slacken