galley slave vs slave

galley slave and slave both are nouns.

Word	Noun	Adjective	Verb	Adverb
galley slave	Yes	No	No	No
slave	Yes	No	Yes	No

As nouns, slave is a hypernym of galley slave; that is, slave is a word with a broader meaning than galley slave:

galley slave (noun)	slave (noun)
a slave condemned to row in a galley	a person who is owned by someone
a laborer who is obliged to do menial work	someone who works as hard as a slave
	someone entirely dominated by some influence or person

galley slave (verb)	slave (verb)
	work very hard, like a slave