idle vs groundless

idle is a noun but groundless is not a noun.

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

As adjectives, idle and groundless are synonyms defined as:

Other synonyms of idle include baseless, unfounded, unwarranted, wild.

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

idle (adjective)	groundless (adjective)
not in action or at work	without a basis in reason or fact
silly or trivial
not in active 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)	groundless (verb)
run disconnected or idle
be idle; exist in a changeless situation