slack‧en [intransitive and transitive] written
1 also slacken off
to gradually become slower, weaker, less active etc, or to make something do this:
The heavy rain showed no signs of slackening off.
slacken your pace/speed (=go or walk more slowly)
Guy slackened his pace as he approached the gate.
to make something looser, or to become looser [≠ tighten]:
He did not let her go, but his grip on her slackened.