fast buck vs net income

fast buck and net income both are nouns.

Word	Noun	Adjective	Verb	Adverb
fast buck	Yes	No	No	No
net income	Yes	No	No	No

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

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

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