Slack1  —adj. 1 (of rope etc.) Not taut. 2 inactive or sluggish. 3 negligent, remiss. 4 (of tide etc.) Neither ebbing nor flowing. —n. 1 slack part of a rope (haul in the slack). 2 slack period. 3 (in pl.) Informal trousers. —v. 1 loosen (rope etc.). 2 colloq. Take a rest, be lazy.  slack off 1 loosen. 2 (also slack up) reduce one's level of activity; reduce speed.  slackness n. [old english]