'downstairs' originates from the combination of 'down' and 'stairs', where 'down' meant 'toward a lower place' and 'stairs' referred to 'a set of steps'.

'downstairs' evolved from the Old English 'dūne' (down) and 'stæger' (stairs), eventually becoming the modern English word 'downstairs'.

Initially, it meant 'toward a lower place via steps', and this meaning has largely remained the same in modern usage.