Fill in the gap.
The Lord’s Prayer is probably the most famous passage from the whole bible. Certainly in the UK where many folks grew up saying it every day in school. But the funny thing is we don’t really know what should fill the gap.
The greek word is ἐπιούσιος (epi-OO-sea-os). Which is a very odd word. It doesn’t appear in other greek literature, and it doesn’t appear elsewhere in the gospels except in the two versions of the Lord’s prayer (Matt 6:9–13 and Luke 11:2–4). So we have no absolute way of knowing what it might mean.
But it seems to be pretty obvious. The Greek splits into two parts: “ἐπι-” (epi) is a common prefix in Greek that means “upon”, or “over”, and by analogy “beyond”, “more-than”. We have it in our word “epidemic”1 where “-demic” is again related to a Greek root meaning people (c.f. words such as demographics or democracy). The second part of the word “-ούσιος” is a common morpheme in Greek with meanings of “substance”. Hence the debate in the early church between whether Jesus and God were “homo-ousios”: of the same substance.
So using this analysis, we get “Give us this day our super-substantial bread”. And this is exactly how it was translated in the Latin Vulgate (“supersubstantialem”), and in some of the earliest English bibles. But, of course, if you are a protestant, “super-substantial bread” is unacceptably reminiscent of “trans-substantial bread”. And Jesus praying a Catholic prayer? Unconscionable!
Instead you can kind-of make a contrived Greek derivation through a misspelling and odd contracted verb form of ἔπειμι (EP-eye-me) “to come upon”, which is often used along with the word for day (“ἡμέρα” herm-EH-ra) to mean “on the following day” (if you’ve read much of the gospels you’ll recognize that phrase, that’s what it is translating). So it could be that we are meant to read “give us this day our following day’s bread”2, with a spelling mistake and a missing word (the missing word isn’t uncommon, it must be said). From there with a squint we could end up with “daily”. Phew. We’ve avoided sounding Catholic! Hoorah! Let’s not mention the fact that there are perfectly reasonable ways of saying “daily” in NT Greek (such as “κατά ἡμέρα” and “πᾶς ἡμέρα” both using the word for day “ἡμέρα”), so that this construction would be an obtusely strange way to say “daily”.
So early protestant translators used “daily”, even though there is very poor evidence for it3. The King James Version, and practically every English version since has used “daily”. I can’t find who originally used this form of words. It is present in Luther’s 1545 version (“täglich Brot”), was written back into the lexicons, and has passed into most other languages from there (“pain quotidien” in the French Louis Segond, for example, and “日々の食物” in the Japanese Living Bible [specially for Sabio]).
So we’ve ended up with “daily” bread. More through historic accident and sectarianism than through good Greek.
So let’s use the obvious Greek parsing “super-substantial”, via a more common English phrase “supernatural” (which is further from the Greek, of course, I’m not suggesting the author meant supernatural, just that supernatural is the nearest reasonably common word in English). We get the Lords prayer (following Matt):
Our father in heaven:
May your name be holy;
May your kingdom come;
May your will be done,
As in the heavens4, so upon the earth.
Give us today our supernatural bread.
Forgive us our sins, even as we forgive sinners5.
Do not bring us to temptation, but from hardships draw us to yourself.6
—Matt 6:9-13 (tr. mine)
So all together we have a prayer which is about the life in the new Kingdom. It doesn’t talk about prosaic things. And using “daily bread” stands out therefore, where “supernatural bread” works fine.
But of course, when you have such an iconic translation, the translation becomes more important than the ‘original’ (or even first 1500 years of) meaning of the text.
 My first thought for an example was “epididymis” which is the coiled tube that pipes sperm out from the testicle. It has a more amusing derivation “epi-” meaning “beyond”, of course, but “-didymis” is also greek, and in fact you may know it from a character in the NT, Didymus Thomas. Didymus means “twins”. So the epididymis, is “beyond the *nudge-nudge-wink-wink* twins”.
 Jerome, the translator of the Vulgate, mentions that the (now lost) Gospel of the Hebrews had “bread of the morrow” here, so the sense of the following day’s bread is not entirely new. But Jerome concludes the “of the morrow” translation doesn’t work in context and makes little sense. He wisely goes with the Greek. He doesn’t mention “daily” as a possible translation.
 Interestingly, the Syriac translation (Pesshita) of the NT uses “neccesary”, although it isn’t clear what parsing of the Greek they were using to arrive at that. This is sometimes pointed to as supporting evidence for the use of the “daily”, though that is a huge stretch even then, since the Syriac used is definitely not that for “daily” – as in the Greek, there is a perfectly normal way of saying “daily”, used elsewhere in the Pesshita, but it is not used here.
 I’ve taken a liberty here of pluralizing the Greek singular word ‘heaven’. The plural form is very common as well, both in Greek and Hebrew and doesn’t seem to have much of a semantic difference. Here it just reads more smoothly.
 My translation of ‘sin’ here is a bit sneaky. Sin would be correct in Luke’s version, but in Matt he uses the normal word for debts. It is pretty certain, however, that it is being used metaphorically, to mean spiritual debts, and this is the sense in which Luke obviously takes it. Not many folks would argue that the gospel writers want you to understand that Jesus is talking monetary debts here. Particularly since Matt 6:14 goes on to have Jesus explain that he was talking about sin. Of course, if Jesus were to be taken as asking you to pray for the forgiveness of your monetary debts, then it would dent my suggestion that the Lords prayer is all about the kingdom.
 The “For thine is the Kingdom” bit at the end is not considered authentic. It is only found in some text traditions and has numerous variations in other texts, so I have omitted it in accordance with the recommendation of Nestle-Aland. — I’m sure that would be yet another reason why conservatives hate the NA text.