slackness n. + Add to list US //ˈsɫæknəs// UK //slˈæknəs// slack·ness SynonymsEtymology Synonyms inanitionlassitudelaxitylaxnesslethargyremissnessslackOriginMEslacnesse→ENslackness From Middle English slacnesse, slakenes, slaknesse; equivalent to slack + -ness. Compare Old English slæcness.