The Collaborative International Dictionary of English v.0.48:

dumper \dumper\ n.
   same as dump truck.

   Syn: dump truck.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

dumper
    n 1: truck whose contents can be emptied without handling; the
         front end of the platform can be pneumatically raised so
         that the load is discharged by gravity [syn: dump truck,
         dumper, tipper truck, tipper lorry, tip truck,
         tipper]