slacken (
http://definr.com/slacken)
v 1: become slow or slower; "Production slowed" [syn:
slow, {slow
down},
slow up,
slack]
2: make less active or fast; "He slackened his pace as he got
tired" [syn:
slack,
slack up]
3: become looser or slack; "the rope slackened"
4: make slack as by lessening tension or firmness [syn:
remit]