'indoor' originates from the combination of 'in' and 'door', where 'in' meant 'inside' and 'door' referred to an entrance or passageway.

'indoor' evolved from the Old English word 'inn' meaning 'inside' and 'duru' meaning 'door', eventually becoming the modern English word 'indoor'.

Initially, it meant 'inside the door', but over time it evolved into its current meaning of 'situated or occurring inside a building'.