idle vs unused

idle is a noun but unused is not a noun.

Word	Noun	Adjective	Verb	Adverb
idle	Yes	Yes	Yes	No
unused	No	Yes	No	No

As adjectives, idle and unused are synonyms defined as:

idle (noun)	unused (noun)
the state of an engine or other mechanism that is idling

idle (adjective)	unused (adjective)
not in action or at work	not yet used or soiled
silly or trivial	not in active use
not in active use	not yet put into use
lacking a sense of restraint or responsibility
not having a job
not yielding a return
without a basis in reason or fact

idle (verb)	unused (verb)
run disconnected or idle
be idle; exist in a changeless situation