slackness

(noun) weakness characterized by a lack of vitality or energy. Synonyms: inanition, lassitude, lethargy.

(noun) the quality of being loose (not taut). Synonyms: slack. Example: "He hadn't counted on the slackness of the rope."

(noun) the quality of being lax and neglectful. Synonyms: laxness, laxity, remissness.