The word "till" actually predates the word "until" - the prefix "un" was added later. Sometime in the 18th century, the usage "'till" (note the apostrophe) became common, later to be replaced by "'til". The former is now considered nonstandard, but the latter has become accepted although it is etymologically incorrect.
Huh. But in the 18th century, was till actually used to mean until, or did it refer only to digging up earth? Cuz unless it was used to mean until, I'd have to think that 'til would be the only proper abbreviation.