'Southall' comes from the Anglo-Saxon for the southern corner.
That is, the corner of a wood or of farmland. Its partner is Northolt to the north, long called Northall, while Southall itself was once known as Southolt. Early records give Suhaull in 1198 and Sudhale in 1204, and the Sheldon tapestry of the late 1580s shows it as Southold.