1. To make or become slower; slow down: The runners slackened their pace. Air speed slackened.

2. To make or become less tense, taut, or firm; loosen: I slackened the line to let the fish swim. The sail's sheet slackened as the wind died.

3. To make or become less vigorous, intense, or severe; ease: slacken discipline; afraid that morale might slacken.