The Collaborative International Dictionary of English v.0.48:

Repoussage \Re*pous`sage"\, n. [F. See Repouss['e].] (Art)
   Art or process of hammering out or pressing thin metal from
   the reverse side: (1) in producing repouss['e] work; (2) in
   leveling up any part of an etched plate that has been worked
   so as to cause a depression.
   [Webster 1913 Suppl.]