Great Britain vs muffin man

Great Britain and muffin man both are nouns.

Word	Noun	Adjective	Verb	Adverb
Great Britain	Yes	No	No	No
muffin man	Yes	No	No	No

Great Britain (noun)	muffin man (noun)
an island comprising England and Scotland and Wales	formerly an itinerant peddler of muffins
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