U.S. vs Last Frontier

U.S. and Last Frontier both are nouns.

Word	Noun	Adjective	Verb	Adverb
U.S.	Yes	No	No	No
Last Frontier	Yes	No	No	No

U.S. (noun)	Last Frontier (noun)
the executive and legislative and judicial branches of the federal government of the United States	a state in northwestern North America; the 49th state admitted to the union
North American republic containing 50 states - 48 conterminous states in North America plus Alaska in northwest North America and the Hawaiian Islands in the Pacific Ocean; achieved independence in 1776