Source : Webster's Revised Unabridged Dictionary (1913)

Rockaway \Rock"a*way\, [Probably from Rockaway beach, where it
   was used.]
   Formerly, a light, low, four-wheeled carriage, with standing
   top, open at the sides, but having waterproof curtains which
   could be let down when occasion required; now, a somewhat
   similar, but heavier, carriage, inclosed, except in front,
   and having a door at each side.