second-string vs secondary

second-string is not a noun while secondary is a noun.

Word	Noun	Adjective	Verb	Adverb
second-string	No	Yes	No	No
secondary	Yes	Yes	No	No

second-string (noun)	secondary (noun)
	coil such that current is induced in it by passing a current through the primary coil
	the defensive football players who line up behind the linemen

second-string (adjective)	secondary (adjective)
being a replacement or substitute for a regular member of a team	being of second rank or importance or value; not direct or immediate
	depending on or incidental to what is original or primary
	inferior in rank or status
	not of major importance
	belonging to a lower class or rank