"A screw cut on a solid, of such form that if any plane be taken through its longitudinal axis, the intersections of the plane by the perimeter are arcs of the pitch-circle of a wheel into which the screw is intended to work. It is so named from having been first employed by Mr. Hindley of York in England." —Whitney, 1889
The hourglass shape of the screw increases the bearing area and therefore reduces wear.
William Dwight Whitney, PhD, LLD The Century Dictionary: An Encyclopedic Lexicon of the English Language (New York, NY: The Century Co., 1895) 5420