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]