Synonyms for Slackness (same or very similar meaning)
WordNet sense 1 (weakness characterized by a lack of vitality or energy):
inanition, lassitude, lethargy
WordNet sense 2 (the condition of being loose (not taut)):
slack
WordNet sense 3 (the quality of being lax and neglectful):
laxity, laxness, remissness
From the ODE community, based on WordNet
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