The compound "stock-still" is a tautological intensifier, a common feature in English where two words of similar meaning are combined for emphasis. The word "stock," originating from a Proto-Germanic root meaning "tree trunk" or "post," evokes the image of something utterly fixed and rigid, like a stake driven into the ground. When paired with "still," from an Old English word meaning motionless or calm, the first element ("stock") acts as a powerful amplifier, transforming "still" from a general descriptor into an absolute state of complete immobility. The logic is vividly literal: to be as motionless as a wooden post.

Structure: stock (sense) + still (sense) - stock: From Old English stocc (tree trunk, post, stake). In this compound, it functions as an intensifier, drawing on the metaphorical sense of something as fixed and immovable as a wooden post. - still: From Old English stille (motionless, quiet, at rest). It functions as the core adjective describing the state of absence of motion.