The Collaborative International Dictionary of English v.0.48:

Tombac \Tom"bac\, n. [Pg. tambaca,tambaque, fr. Malay tamb[=a]ga
   copper; cf. Skr. t[=a]mraka; cf. F. tombac.] (Metal.)
   An alloy of copper and zinc, resembling brass, and containing
   about 84 per cent of copper; -- called also German brass or
   Dutch brass. It is very malleable and ductile, and when
   beaten into thin leaves is sometimes called Dutch metal.
   The addition of arsenic makes white tombac. [Written also
   tombak, and tambac.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

tombac
    n 1: an alloy of copper and zinc (and sometimes arsenic) used to
         imitate gold in cheap jewelry and for gilding [syn:
         tombac, tombak, tambac]