'first-floor' originates from the combination of 'first,' meaning 'preceding all others in time or order,' and 'floor,' meaning 'the lower surface of a room.'

'first-floor' has been used in English since the 19th century to denote the ground level of a building, especially in American English.

Initially, it meant 'the first level above the ground,' but in American English, it evolved to mean 'the ground level.'