He all but ran out of the building
If I understand it correctly (English is not my native language), this sentence means "he ran out of the building", and the "all but" is here to stress the fact that he didn't do anything else (besides running).
That being said, the first time I came across this construction, I understood it the other way round. Eventually, I inferred from context the above meaning, but even today, I still have a feeling that it's somewhat illogical.
Where does this construction come from (and what does it really mean)?