slackness in WordNet English dictionary

1. weakness characterized by a lack of vitality or energy
   inanition  lassitude  lethargy  slackness 
   
2. the quality of being lax and neglectful
   laxness  laxity  remissness  slackness 
   
3. the quality of being loose (not taut)
   "he hadn't counted on the slackness of the rope"
   slack  slackness