sit back vs relax

sit back and relax both are verbs.

Word	Noun	Adjective	Verb	Adverb
sit back	No	No	Yes	No
relax	No	No	Yes	No

As verbs, relax is a hypernym of sit back; that is, relax is a word with a broader meaning than sit back:

Other hypernyms of sit back include decompress, loosen up, slow down, unbend, unwind.

sit back (verb)	relax (verb)
settle into a comfortable sitting position	become less tense, rest, or take one's ease
be inactive or indifferent while something is happening	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