An expression combining the English words all and but, which together have meanings such as "almost", or "very nearly", "to the point of, but not entirely." These should always be followed by a verb or an adjective which indicates a completed or extreme state.

Examples:

all but exhausted -- very, very tired

all but impossible -- very, very difficult

This expression dates back to the 1600's, when English didn't really make a whole lot of sense. It is still in use 400 years later, when English still doesn't make a lot of sense.