Wordnet 3.0
VERB (1)
1. 
 make less active or fast; 
- Example: "He slackened his pace as he got tired"- Example: "Don't relax your efforts now"[syn: slack, slacken, slack up, relax]
WordNet (r) 3.0 (2006):
slack up
    v 1: make less active or fast; "He slackened his pace as he got
         tired"; "Don't relax your efforts now" [syn: slack,
         slacken, slack up, relax]