fast buck vs profits

fast buck and profits both are nouns.

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

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

fast buck: quick or easy earnings
profits: 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 earnings, lucre, net, net income, net profit, profit.

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