Great Britain vs mercer

Great Britain and mercer both are nouns.

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

Great Britain (noun)	mercer (noun)
an island comprising England and Scotland and Wales	a dealer in textiles (especially silks)
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