loosen up vs sit back

loosen up and sit back both are verbs.

Word	Noun	Adjective	Verb	Adverb
loosen up	No	No	Yes	No
sit back	No	No	Yes	No

As verbs, sit back is a hyponym of loosen up; that is, sit back is a word with a more specific, narrower meaning than loosen up:

Other hyponyms of loosen up include veg out, vegetate, take it easy.

loosen up (verb)	sit back (verb)
become less tense, rest, or take one's ease	settle into a comfortable sitting position
cause to feel relaxed	be inactive or indifferent while something is happening
make one's body limber or suppler by stretching, as if to prepare for strenuous physical activity
cause to become unblocked
become less tense, less formal, or less restrained, and assume a friendlier manner