Noah Webster's New International Dictionary of the English Language

1. (v.) To become slack; to be made less tense, firm, or rigid; to decrease in tension; as, a wet cord slackens in dry weather.

2. (v.) To be remiss or backward; to be negligent.

3. (v.) To lose cohesion or solidity by a chemical combination with water; to slake; as, lime slacks.

4. (v.) To abate; to become less violent.

5. (v.) To lose rapidity; to become more slow; as, a current of water slackens.

6. (v.) To languish; to fail; to flag.

7. (v.) To end; to cease; to desist; to slake.

8. (v. t.) To render slack; to make less tense or firm; as, to slack a rope; to slacken a bandage.

9. (v. t.) To neglect; to be remiss in.

10. (v. t.) To deprive of cohesion by combining chemically with water; to slake; as, to slack lime.

11. (v. t.) To cause to become less eager; to repress; to make slow or less rapid; to retard; as, to slacken pursuit; to slacken industry.

12. (v. t.) To cause to become less intense; to mitigate; to abate; to ease.

13. (n.) A spongy, semivitrifled substance which miners or smelters mix with the ores of metals to prevent their fusion.