earnings vs quick buck

earnings and quick buck both are nouns.

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

As nouns, quick buck is a hyponym of earnings; that is, quick 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)
quick buck: quick or easy earnings

earnings (noun)	quick 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)