laxness vs slackness

laxness and slackness both are nouns.

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

As nouns, laxness and slackness are synonyms defined as:

Other synonyms of laxness include laxity, remissness.

laxness (noun)	slackness (noun)
the quality of being lax and neglectful	the quality of being loose (not taut)
the condition of being physiologically lax	the quality of being lax and neglectful
	weakness characterized by a lack of vitality or energy