The adverbial phrase all but (no need to hyphenate it) means almost, nearly, or on the verge of. It signals that the following word is almost but not quite the case. For example, if I say, I all but ran to the door, it means I walked very fast to the door but did not run. Or if I say I’m all but finished making dinner, it means I’m putting the finishing touches on the meal.
Of course but is sometimes a synonym of except, so all but can also mean all except—e.g., I read all but the last chapter.
Examples
Pennsylvania’s Democratic boss is all but declaring victory in his state’s 2012 Senate contest. [Politico]
Freestanding statues all but disappeared from Western art after the fifth century. [History of Art: The Western Tradition]
The Age says the carbon tax deal is all but done. [The Australian]
[W]ith that date just 11 days away and no talks taking place, getting an agreement by then looks all but impossible. [Globe and Mail]
In June, net job creation all but stopped. [Financial Times]