From The Collaborative International Dictionary of English v.0.48 :
Slackness \Slack"ness\, n.
The quality or state of being slack.
[1913 Webster]
From WordNet (r) 3.0 (2006) :
slackness
n 1: weakness characterized by a lack of vitality or energy
[syn: inanition, lassitude, lethargy, slackness]
2: the quality of being loose (not taut); "he hadn't counted on
the slackness of the rope" [syn: slack, slackness]
3: the quality of being lax and neglectful [syn: laxness,
laxity, remissness, slackness]