slack vs lessen

slack is a noun but lessen is not a noun.

Word	Noun	Adjective	Verb	Adverb
slack	Yes	Yes	Yes	No
lessen	No	No	Yes	No

As verbs, lessen is a hypernym of slack; that is, lessen is a word with a broader meaning than slack:

Other hypernyms of slack include decrease, minify.

As verbs, lessen is a hypernym of slack; that is, lessen is a word with a broader meaning than slack:

Other hypernyms of slack include decrease, minify.

As verbs, lessen is a hypernym of slack; that is, lessen is a word with a broader meaning than slack:

Other hypernyms of slack include decrease, diminish, fall.

slack (noun)	lessen (noun)
a cord or rope or cable that is hanging loosely
the quality of being loose (not taut)
a stretch of water without current or movement
a soft wet area of low-lying land that sinks underfoot
a noticeable deterioration in performance or quality
dust consisting of a mixture of small coal fragments and coal dust and dirt that sifts out when coal is passed over a sieve

slack (adjective)	lessen (adjective)
not tense or taut
lacking in rigor or strictness
flowing with little speed as e.g. at the turning of the tide

slack (verb)	lessen (verb)
release tension on	decrease in size, extent, or range
make less active or fast	wear off or die down
make less active or intense	make become smaller
become slow or slower
cause to heat and crumble by treatment with water
become less in amount or intensity
be inattentive to, or neglect
avoid responsibilities and work, be idle