earnings vs fast buck

earnings and fast buck both are nouns.

Word	Noun	Adjective	Verb	Adverb
earnings	Yes	No	No	No
fast buck	Yes	No	No	No

As nouns, fast buck is a hyponym of earnings; that is, fast buck is a word with a more specific, narrower meaning than earnings:

earnings: the excess of revenues over outlays in a given period of time (including depreciation and other non-cash expenses)
fast buck: quick or easy earnings

earnings (noun)	fast buck (noun)
something that remunerates	quick or easy earnings
the excess of revenues over outlays in a given period of time (including depreciation and other non-cash expenses)