- To make or become slower; slow down: The runners slackened their pace. Air speed slackened. (verb-transitive)
- 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. (verb-transitive)
- To make or become less vigorous, intense, or severe; ease: slacken discipline; afraid that morale might slacken. (verb-transitive)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.