"slackness" in the noun sense
1. inanition, lassitude, lethargy, slackness
weakness characterized by a lack of vitality or energy
2. slack, slackness
the quality of being loose (not taut
"he hadn't counted on the slackness of the rope"
3. laxness, laxity, remissness, slackness
the quality of being lax and neglectful
Source: WordNet® (An amazing lexical database of English)
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