slackness vs neglectfulness

slackness and neglectfulness both are nouns.

Word	Noun	Adjective	Verb	Adverb
slackness	Yes	No	No	No
neglectfulness	Yes	No	No	No

As nouns, neglectfulness is a hypernym of slackness; that is, neglectfulness is a word with a broader meaning than slackness:

slackness: the quality of being lax and neglectful
neglectfulness: the trait of neglecting responsibilities and lacking concern

Other hypernyms of slackness include neglect, negligence.

slackness (noun)	neglectfulness (noun)
the quality of being loose (not taut)	the trait of neglecting responsibilities and lacking concern
the quality of being lax and neglectful
weakness characterized by a lack of vitality or energy