debitor vs fly-by-night

debitor and fly-by-night both are nouns.

Word	Noun	Adjective	Verb	Adverb
debitor	Yes	No	No	No
fly-by-night	Yes	Yes	No	No

As nouns, fly-by-night is a hyponym of debitor; that is, fly-by-night is a word with a more specific, narrower meaning than debitor:

debitor: a person who owes a creditor; someone who has the obligation of paying a debt
fly-by-night: a debtor who flees to avoid paying

Other hyponyms of debitor include deadbeat, defaulter, mortgager, mortgagor.

debitor (noun)	fly-by-night (noun)
a person who owes a creditor; someone who has the obligation of paying a debt	a debtor who flees to avoid paying

debitor (adjective)	fly-by-night (adjective)
	ephemeral
	(of businesses and businessmen) untrustworthy or undependable