thinness vs fluidness

thinness and fluidness both are nouns.

Word	Noun	Adjective	Verb	Adverb
thinness	Yes	No	No	No
fluidness	Yes	No	No	No

As nouns, fluidness is a hyponym of thinness; that is, fluidness is a word with a more specific, narrower meaning than thinness:

Other hyponyms of thinness include fluidity, liquidity, liquidness, runniness, wateriness.

thinness (noun)	fluidness (noun)
the property of having little body fat	the property of flowing easily
relatively small dimension through an object as opposed to its length or width	a changeable quality
a consistency of low viscosity
the property of being very narrow or thin
the property of being scanty or scattered; lacking denseness
The property of lacking substance or strength.