Etymology
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slacken (v.)

early 15c., from slack (adj.) + -en (1). Related: Slackened; slackening.

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Definitions of slacken

slacken (v.)
become looser or slack;
the rope slackened
slacken (v.)
make slack as by lessening tension or firmness;
Synonyms: remit
slacken (v.)
make less active or fast;
He slackened his pace as he got tired
Synonyms: slack / slack up / relax
slacken (v.)
become slow or slower;
Synonyms: slow / slow down / slow up / slack
From wordnet.princeton.edu