supergrass vs Great Britain

supergrass and Great Britain both are nouns.

Word	Noun	Adjective	Verb	Adverb
supergrass	Yes	No	No	No
Great Britain	Yes	No	No	No

supergrass (noun)	Great Britain (noun)
a police informer who implicates many people	an island comprising England and Scotland and Wales
	a monarchy in northwestern Europe occupying most of the British Isles; divided into England and Scotland and Wales and Northern Ireland; `Great Britain' is often used loosely to refer to the United Kingdom