slack ‧ en /ˈslækən/ BrE AmE verb [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.
2 . to make something looser, or to become looser OPP tighten :
He did not let her go, but his grip on her slackened.