tidy sum vs inundation

tidy sum and inundation both are nouns.

Word	Noun	Adjective	Verb	Adverb
tidy sum	Yes	No	No	No
inundation	Yes	No	No	No

As nouns, inundation is a hyponym of tidy sum; that is, inundation is a word with a more specific, narrower meaning than tidy sum:

Other hyponyms of tidy sum include deluge, flood, torrent, haymow.

tidy sum (noun)	inundation (noun)
(often followed by `of') a large number or amount or extent	the rising of a body of water and its overflowing onto normally dry land
	an overwhelming number or amount