piercing vs keen

piercing is not a noun while keen is a noun.

Word	Noun	Adjective	Verb	Adverb
piercing	No	Yes	No	No
keen	Yes	Yes	Yes	No

As adjectives, piercing and keen are synonyms defined as:

piercing and keen: having or demonstrating ability to recognize or draw fine distinctions

Other synonyms of piercing include acute, discriminating, incisive, knifelike, penetrating, penetrative, sharp.

As adjectives, piercing and keen are synonyms defined as:

Other synonyms of piercing include cutting, knifelike, lancinate, lancinating, stabbing.

piercing (noun)	keen (noun)
	a funeral lament sung with loud wailing

piercing (adjective)	keen (adjective)
having or demonstrating ability to recognize or draw fine distinctions	excellent
painful as if caused by a sharp instrument	intense or sharp
	having or demonstrating ability to recognize or draw fine distinctions
	having a sharp cutting edge or point
	painful as if caused by a sharp instrument

piercing (verb)	keen (verb)
	express grief verbally