slacken

from The American Heritage® Dictionary of the English Language, 5th Edition.

• transitive & intransitive verb To make or become slower; slow down.
• transitive & intransitive verb To make or become less tense, taut, or firm; loosen.
• transitive & intransitive verb To make or become less vigorous, intense, or severe; ease.

from The Century Dictionary.

• To become slack.
• To become less active; fall off: as, trade slackened; the demand slackens; prices slacken
• To become remiss or neglectful, as of duty.
• To make slack or slacker.
• 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.
• To be or become remiss in or neglectful of; remit; relax: as, to slacken labor or exertion.
• noun 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.

from the GNU version of the Collaborative International Dictionary of English.

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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• verb intransitive To gradually decrease in intensity or tautness; to become slack.
• verb transitive To make slack, less taut, or less intense.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• verb make less active or fast
• verb make slack as by lessening tension or firmness
• verb become looser or slack
• verb become slow or slower