fast buck vs earnings

fast buck and earnings both are nouns.

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

As nouns, earnings is a hypernym of fast buck; that is, earnings is a word with a broader meaning than fast buck:

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

Other hypernyms of fast buck include lucre, net, net income, net profit, profit, profits.

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