[Stingray]

Greek Word: ἐπιούσιος Transliterated Word: epiousios Root: from 1897a; Definition: for the coming day, for subsistence:-- Try reading it like this: "Give us our bread for the coming day." The context of what Jesus means here is set forth in Matthew 6, verses 25-34, especially verse 34. After teaching them how to pray and what to pray for, He tells them not to worry about tomorrow. I believe the context makes the meaning clear.

[BwanaNdege]

Look at this in the context of the manna in the wilderness. The Hebrews were commanded to collect only enough for that day, except, prior to the Sabbath, they were to collect two days worth. Exodus 16:1-30

[Fedora]

What Stingray says is also how Liddell and Scott explain the derivation of the root word: from "he epiousa [hemera]" (vs. ousios as Msgr. Pope has it) where hemera is the word for "day" and epi is from the prefix for "on"/"upon"/"at the time of"/etc. (depending on the case and context). If you follow the first link I gave (to Strong's commentary on epiousa), it relates the etymology to epieper and gives several occurrences of the latter with multiple lexical references.