Source : Webster's Revised Unabridged Dictionary (1913)

Pickaback \Pick"a*back`\, adv.
   On the back or shoulders; as, to ride pickback. [Written also
   {pickapack}, {pickback}, and {pickpack}.]

Source : WordNet®

pickaback
     adv 1: on a railroad flatcar; "the trailer rode piggyback across
            the country" [syn: {piggyback}, {pig-a-back}]
     2: on the back or shoulder or astraddle on the hip; "she
        carried her child piggyback" [syn: {piggyback}, {pig-a-back}]