"slacken" in the verb sense
1. slow, slow down, slow up, slack, slacken
become slow or slower
2. slack, slacken, slack up, relax
make less active or fast
"He slackened his pace as he got tired"
"Don't relax your efforts now"
become looser or slack
"the rope slackened"
4. slacken, remit
make slack as by lessening tension or firmness
Source: WordNet® (An amazing lexical database of English)
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