slacken

American Heritage® Dictionary of the English Language, Fourth Edition

1. v. To make or become slower; slow down: The runners slackened their pace. Air speed slackened.
2. v. To make or become less tense, taut, or firm; loosen: I slackened the line to let the fish swim. The tension in the board room finally slackened.
3. v. To make or become less vigorous, intense, or severe; ease: slacken discipline; afraid that morale might slacken.

Century Dictionary and Cyclopedia

1. To become slack. To become less tense, firm, or rigid: as, a wet cord slackens in dry weather.
2. To become less active; fall off: as, trade slackened; the demand slackens; prices slacken
3. To become remiss or neglectful, as of duty.
4. To make slack or slacker. To lessen or relieve the tension of; loosen; relax: as, to slacken a bandage, or an article of clothing.
5. To abate; moderate; lessen; diminish the intensity, severity, rate, etc., of; hence, to mitigate; assuage; relieve: as, to slacken one's pace; to slacken cares.
6. To be or become remiss in or neglectful of; remit; relax: as, to slacken labor or exertion.
7. n. The slags or cinder from previous fusions, used in smelting operations to mix with natural ores and to retard fusion of the ores until reduction shall have proceeded to the desired point.

Wiktionary

1. v. intransitive To gradually decrease in intensity or tautness; to become slack.
2. v. transitive To make slack, less taut, or less intense.

GNU Webster's 1913

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

WordNet 3.0

1. v. make less active or fast
2. v. make slack as by lessening tension or firmness
3. v. become looser or slack
4. v. become slow or slower