rag day vs United Kingdom of Great Britain and Northern Ireland

rag day and United Kingdom of Great Britain and Northern Ireland both are nouns.

Word	Noun	Adjective	Verb	Adverb
rag day	Yes	No	No	No
United Kingdom of Great Britain and Northern Ireland	Yes	No	No	No

rag day (noun)	United Kingdom of Great Britain and Northern Ireland (noun)
a day on which university students hold a rag	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