slave vs break one's back

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

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

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

Other synonyms of slave include buckle down, knuckle down.

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

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