sky-high

The compound "sky-high" is a relatively modern English formation, emerging in the 19th century. It combines the noun "sky," referring to the expanse above the earth, with the adjective "high," creating a vivid, figurative expression for an extreme degree. The logic is transparently metaphorical: something elevated to the level of the sky is at its maximum possible height. This literal imagery evolved into its modern abstract meanings, describing anything—from prices to spirits—that reaches an exceptionally great level.

Structure: sky (the sky) + high (elevated) - sky: From Old Norse ský (cloud), of Proto-Germanic origin. Functions as a noun root, here used attributively to denote the domain of great height. - high: From Old English hēah (of great vertical extent), of Proto-Germanic origin. Functions as an adjective/adverb root, denoting elevation.