grin vs smirk

grin and smirk both are nouns.

Word	Noun	Adjective	Verb	Adverb
grin	Yes	No	Yes	No
smirk	Yes	No	Yes	No

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

grin: a facial expression characterized by turning up the corners of the mouth; usually shows pleasure or amusement
smirk: a smile expressing smugness or scorn instead of pleasure

Other hyponyms of grin include simper.

grin (noun)	smirk (noun)
a facial expression characterized by turning up the corners of the mouth; usually shows pleasure or amusement	a smile expressing smugness or scorn instead of pleasure

grin (verb)	smirk (verb)
to draw back the lips and reveal the teeth, in a smile, grimace, or snarl	smile affectedly or derisively