knuckle down vs slave

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

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

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

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

knuckle down (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

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