From it article it seems to be a new coinage in latin at the time of the translation. It makes sense only in the logic of theology so don't think too rationally about it. some clues from the article:
> Taken literally (epi-ousios: "super-essential"), it refers directly to the Bread of Life, the Body of Christ, the "medicine of immortality," without which we have no life within us.
> In all languages that traditionally Eastern Christians use—Greek, Slavonic, and all the Arabic languages... the best translation would be: "Give us today the bread of tomorrow". Give us today the bread of the coming age, the bread that when you eat it, you can never die. What is the food of the coming age? It's God himself, God's word, God's Son, God's lamb, God's bread, which we already have here on earth, on earth, before the second coming. So what we're really saying is, "Feed us today with the bread of the coming age", because we are taught by Jesus not to seek the bread that perishes, but the bread that, you eat it, you can never die
In the Catholic rite, the bread _becomes_ the body of the Christ during Eucharist, a process known as _transubstantiation_ (meanwhile in protestant liturgy, it _contains_ the body of the Christ, a process known as _consubstantiation_). In any case, the bread is more than just bread, it's the symbol of the sacrifice made by the Christ to save Humanity and renew the alliance made with God. I think (I'm not a doctor of canon law by the way) that's what it means: the bread beyond its substance.
The Catholic theological tradition holds that the "form" remains bread, but the "substance" becomes God. It is explicitly not a symbol, it is God. Many of the laity don't understand this or, even if they do, do not think it a necessary article of faith.
This reminds me of a phrase I've heard many times, but I don't know the origin. "Protestants believe Eucharist is symbolic. Catholics believe the Eucharist is real. Orthodox Christians believe the Eucharist is real because it's symbolic."
The word "symbol" originally meant "where two things come together" as in, two rivers joining into one river. Many Protestants believe that the Eucharist is only a token or reminder of Christ. The Catholics have developed the idea that it materially changes. The Orthodox believe it changes into the Body of Christ, but not in a way you could physically measure.
I'm not a theologian and this is my poor layman's understanding.
It may have developed, but it appears to have developed quite early, if you read the writings of the 1st and 2nd century. It was reading Ignatius of Antioch (or maybe Polycarp) that made me realize how early the Catholic / Orthodox understanding was.
Catholics do not believe you can physically measure the change to the eucharist. The aristotelian understanding and explanation of transubstantiation is not a requirement of the catholic religion as emphasized by the unity of the eastern catholic churches who do hold on to the dogma that the bread and wine are indeed the body and blood of Christ but teach nothing as to how that takes place.
That's kind of the point. No one does or has for at least 1700 years. "Daily" or "Enough for tomorrow/the time after" seems to lead the pack due to its' appearance in an Egyptian shopping list and similar Biblical words.
It must be "soul food". There's material world "below" and there's world of ideas "above". Matter has essence, so it's substantial; and ideas must be "supersubstancial". That "lord's prayer" tells something along this line: "oh, the god who lives above, your name is sacred, your material world reflects your ideas above, give us soul food, save us from duality, ..." (my own interpretation, sounds a bit egyptian).
