slave vs knuckle down

slave is a noun but knuckle down is not a noun.

Word	Noun	Adjective	Verb	Adverb
slave	Yes	No	Yes	No
knuckle down	No	No	Yes	No

As verbs, slave and knuckle down are synonyms defined as:

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

slave (noun)	knuckle down (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)	knuckle down (verb)
work very hard, like a slave	work very hard, like a slave