bottom vs rock bottom

bottom and rock bottom both are nouns.

Word	Noun	Adjective	Verb	Adverb
bottom	Yes	Yes	Yes	No
rock bottom	Yes	No	No	No

As nouns, rock bottom is a hyponym of bottom; that is, rock bottom is a word with a more specific, narrower meaning than bottom:

Other hyponyms of bottom include foot, base.

bottom (noun)	rock bottom (noun)
a cargo ship	the absolute bottom
the fleshy part of the human body that you sit on
the lower side of anything
the lowest part of anything
a depression forming the ground under a body of water
low-lying alluvial land near a river
the second half of an inning; while the home team is at bat

bottom (adjective)	rock bottom (adjective)
situated at the bottom or lowest position
the lowest rank

bottom (verb)	rock bottom (verb)
come to understand
strike the ground, as with a ship's bottom
provide with a bottom or a seat