thus, so: a pointing word that singles out the exact manner (or, rarely, the degree) of something already in view
This is a pointing word, like the English "thus," "so," or "this way." It never carries meaning on its own; it always gestures at something in the surrounding text, either the way an action was just shown or described, or the way it is about to be done.
Almost every time, it marks manner: do it "thus," it happened "so," it was done "in this way." It is at home in instructions ("thus you shall eat it," "thus shall it be done to the man") and in the recurring stunned question about disaster, "Why has the LORD done so to this land?" A handful of times it slides toward degree, pointing not to how something is done but to how far a condition has gone, as when Amnon is asked why he looks "so" wasted morning after morning.
It sits alongside two other pointing adverbs, כֹּה and כֵּן. Where כֹּה is the standard word that introduces quoted speech ("thus says the LORD"), this word leans toward the manner of an action that can be seen or has been carried out.