'southwestern' originates from the combination of 'south' and 'west', where 'south' meant 'the direction toward the point of the horizon 90° clockwise from east' and 'west' meant 'the direction toward the point of the horizon where the sun sets'.

'south' and 'west' combined in Middle English to form 'southwest', which eventually led to the adjective 'southwestern'.

Initially, it meant 'toward the southwest direction', and this meaning has largely remained the same in modern usage.