verb as in lessen
"An' another thing about this pony o' mine, he'll never slack up on a rope," continued Dan.
Slack up, if you please, and let me have all the line you can, without casting off from your body.
He was a medical man, past forty, beginning to slack up a little in his elasticity and strength.
A little later the Coyote obviously had to slack up to keep from running away altogether.
Ill let her run at this rate for a while, and then well slack up.
On this page you'll find 63 synonyms, antonyms, and words related to slack up, such as: abate, curtail, decrease, dilute, diminish, and dwindle.
From Roget's 21st Century Thesaurus, Third Edition Copyright © 2013 by the Philip Lief Group.