lassie vs girl

lassie and girl both are nouns.

Word	Noun	Adjective	Verb	Adverb
lassie	Yes	No	No	No
girl	Yes	No	No	No

As nouns, girl is a hypernym of lassie; that is, girl is a word with a broader meaning than lassie:

Other hypernyms of lassie include fille, miss, missy, young lady, young woman.

lassie (noun)	girl (noun)
a girl or young woman who is unmarried	a young female
	a girl or young woman with whom a man is romantically involved
	a youthful female person
	a friendly informal reference to a grown woman
	a female human offspring