Here is the Etymology, By LSJ Lexicon (Henry Liddell, Robert Scott, Stuart Jones, Robert Mackenzie)
Ἀλήθεια (aléthea [noun]) < ἀληθής (alethés [adjective]): adjective, originally identifying things or events, meaning "one who cannot be hidden", "(who is) real" as opposed to the adjective “(who is) false”. After Homer, the adjective also defined persons, meaning "one who does not deceive, who does not lie", hence “φιλαλήθης” (philalíthes = friend of the truth or who loves the truth). From the adjective “ἀληθής” came the abstract noun “ἀλήθεια” (aléthea = relatived to the meaning of truth), which literally means "state in which nothing is hidden", therefore "real state" —in contrast to “ψεῦδος” (psévdhos = false, lie, hypocrisy, etc [everything that is not real state])— and in the case of persons it means "honesty". The word “ἀλήθεια” was widely used as a term in philosophy, where it usually denotes "a state of knowledge, recollection, memory" as opposed to “λήθη” (léthe = oblivion). Etymologically, the adjective “ἀληθής”, which replaced the word “ἐτεός” (eteós) {I think it has to do with the Greek word alethés, but I believe, according to the possible premologies (because they are not confirmed), that it is the opposite of "κενός" (kenós = empty, from "κενε[F]oς > κεν[F]oς > κενος", i.e., behind what someone claims or does, there is a substance, reality, truth, which pushes him to do what he does.}, is considered a compound with the preffix ἀ- (negative prefix) and the second compound the noun “λῆθος” (Doric λᾶθος) or “λήθη” (forgetfulness), not excluding the direct connection of the adjective with the verb “λήθω” {létho (parallel form of the verb is “λανθάνω” [lantháno])}.
My personal view
My personal view, regarding the concept generally used in the New Testament, has to do with the actual situation that a true Christian experiences. I.e., a Christian learns not to pretend (externalizing his true self), to have faith in the living God (real: alethés = in both senses, perhaps, of the word, i.e. he who speaks the truth, does not pretend, as also, he exists, is alive, and his works are seen in our eyes by which we understand what God represents), not to lie, not to make mistakes, but not to feel like he's being fair, not to be forgotten (rejected) by God (God's gaze, his attention and care, is upon his children; God recognizes you, which means that you live in truth; while others live in a lie, i.e. Away from God and his attribute, his designation, his name [the name is an attribute] which is the truth). So then, the noun "ἀλήθεια" (aléthea) has come, and has taken on another dimension from Christ. Because, no longer does “aléthea” represent a man who looks to please the desires of his flesh, which is a natural thing for a man, and I mean food pleasures, the importance a man wants to feel, wanting to believe in a god, which happens to be a foreign god, pleasure in general, and carnal intercourse with the opposite sex using abuse (God allows monogamy and, I think, that is what the scripture is telling us, within the context of marriage); all these things are natural for man to want, therefore, natural for him to do them, therefore, he who does them is in a truth. But, Christ came and changed that. He changed the truth. Truth now is to be in the will of God in the full meaning of the word "will".
