U.K. vs po-faced

U.K. is a noun but po-faced is not a noun.

Word	Noun	Adjective	Verb	Adverb
U.K.	Yes	No	No	No
po-faced	No	Yes	No	No

U.K. (noun)	po-faced (noun)
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

U.K. (adjective)	po-faced (adjective)
	humorless and disapproving