relax vs slack up

relax and slack up both are verbs.

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

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

Other synonyms of relax include slack, slacken.

relax (verb)	slack up (verb)
become less tense, rest, or take one's ease	make less active or fast
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