fall vs slack

fall and slack both are nouns.

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

As verbs, slack is a hyponym of fall; that is, slack is a word with a more specific, narrower meaning than fall:

fall (noun)	slack (noun)
a sudden drop from an upright position	a cord or rope or cable that is hanging loosely
the act of surrendering (usually under agreed conditions)	the quality of being loose (not taut)
a lapse into sin; a loss of innocence or of chastity	a stretch of water without current or movement
a sudden sharp decrease in some quantity	a soft wet area of low-lying land that sinks underfoot
a free and rapid descent by the force of gravity	a noticeable deterioration in performance or quality
a sudden decline in strength or number or importance	dust consisting of a mixture of small coal fragments and coal dust and dirt that sifts out when coal is passed over a sieve
when a wrestler's shoulders are forced to the mat
a movement downward
a downward slope or bend
the season when the leaves fall from the trees
the time of day immediately following sunset

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

fall (verb)	slack (verb)
occur at a specified time or place	release tension on
decrease in size, extent, or range	make less active or fast
pass suddenly and passively into a state of body or mind	make less active or intense
be born, used chiefly of lambs	become slow or slower
come out; issue	cause to heat and crumble by treatment with water
assume a disappointed or sad expression	become less in amount or intensity
begin vigorously	be inattentive to, or neglect
come as if by falling	avoid responsibilities and work, be idle
be cast down
go as if by falling
fall or flow in a certain way
move downward and lower, but not necessarily all the way
move in a specified direction
descend in free fall under the influence of gravity
slope downward
drop oneself to a lower or less erect position
lose an upright position suddenly
to be given by assignment or distribution
be captured
fall to somebody by assignment or lot; passed
come into the possession of
to be given by right or inheritance
be inherited by
lose office or power
yield to temptation or sin
suffer defeat, failure, or ruin
lose one's chastity
die, as in battle or in a hunt
be due
come under, be classified or included
touch or seem as if touching visually or audibly
fall from clouds