break one's back vs slave

break one's back is not a noun while slave is a noun.

Word	Noun	Adjective	Verb	Adverb
break one's back	No	No	Yes	No
slave	Yes	No	Yes	No

As verbs, break one's back and slave are synonyms defined as:

Other synonyms of break one's back include buckle down, knuckle down.

break one's back (noun)	slave (noun)
	a person who is owned by someone
	someone who works as hard as a slave
	someone entirely dominated by some influence or person

break one's back (verb)	slave (verb)
work very hard, like a slave	work very hard, like a slave