slack up vs relax

slack up and relax both are verbs.

Word	Noun	Adjective	Verb	Adverb
slack up	No	No	Yes	No
relax	No	No	Yes	No

As verbs, slack up and relax are synonyms defined as:

Other synonyms of slack up include slack, slacken.

slack up (verb)	relax (verb)
make less active or fast	become less tense, rest, or take one's ease
	cause to feel relaxed
	make less taut
	become loose or looser or less tight
	make less active or fast
	become less severe or strict
	make less severe or strict
	become less tense, less formal, or less restrained, and assume a friendlier manner