Up to the time or degree that; until.
"He waited unto dawn before departing for his journey."
Up to, indicating a motion towards a thing and then stopping at it.
"The hiker walked up the steep trail all the way unto the summit where he paused to catch his breath."
Unto comes from Middle English and Old English, where it originally meant "against," "toward," or "up to." The word entered modern usage primarily in the sense of "until" or "to," evolving directly from its earlier components.