keen vs piercing

keen is a noun but piercing is not a noun.

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

As adjectives, keen and piercing are synonyms defined as:

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

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

As adjectives, keen and piercing are synonyms defined as:

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

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

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

keen (verb)	piercing (verb)
express grief verbally