'in-depth' originates from the combination of 'in' and 'depth', where 'in' meant 'inside' and 'depth' referred to 'the distance from the top or surface to the bottom'.

'In-depth' evolved from the phrase 'in depth', which was used to describe something that was thoroughly examined or explored.

Initially, it meant 'inside the depth', but over time it evolved into its current meaning of 'comprehensive and thorough'.