The adjective (aionios)is never found until the writings of Plato (427 BC - 347 BC) who only used the word five times, and while he did use this word in the context of eternity, he never used it by itself to mean such.
Why?
The word, in and of itself, does not mean eternal. Whenever he wanted to convey the idea of eternity, he always combined a stronger forced word with it such as aidios. Not once did he ever use aionios by itself to mean endless.
However
Both Plato and Aristotle did use the word by itself to mean temporary.
Dr. J. W. Hanson
Plato, referrring to certain souls in Hades, describes them as being in aionian intoxication.
He does not use the word in the sense of endless is evident from the Phaedon, where he says, "It is a very ancient opinion that souls quitting the world, repair to the infernal regions, and return after that, to live in this world."
"After the aionian intoxication is over, they return to earth, which demonstrates that the word was not used by him as meaning endless."
Aristotle uses the word in the same sense.
"All these things (the earth) seem to be done for her good, in order to maintain safety during her aionios duration, or life.
And still more to the purpose is this quotation concerning God's existence:
"Life and 'an aion continuous and eternal, zoe kai aion sunekes kai aidios." Here the word aidios, (eternal) is employed to qualify aion and impart to it what it had not of itself, the sense of eternal.
So we can see from classic Greek usage that the word aionios meant a temporal period of time and was not used to convey the idea of eternity.
"As the marsh-hen secretly builds on the watery sod, behold I will build me a nest on the greatness of God." -Sidney Lanier_