rock bottom vs bottom

rock bottom and bottom both are nouns.

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

As nouns, bottom is a hypernym of rock bottom; that is, bottom is a word with a broader meaning than rock bottom:

rock bottom (noun)	bottom (noun)
the absolute bottom	a cargo ship
	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

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

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