'sky-high' originates from the English words 'sky' and 'high', where 'sky' refers to the expanse of air over the earth and 'high' means elevated or tall.

'sky-high' has been used in English since the 19th century to describe something that is extremely high or elevated.

Initially, it meant 'as high as the sky', but over time it evolved into its current meaning of 'extremely high or expensive'.