Green’s Dictionary of Slang

(UK Und.) a counterfeit coin made from a mixed metal.

1665		Head Eng. Rogue I 186: Another called Compositum, which is a mixt Metal, and will both touch and cut, but will not endure the fiery test.
1714		A. Smith Lives of Most Noted Highway-men, etc. I 205: What they [i.e. coiners] call Compositum, is a mix’d Metal, which will both touch and cut, but not endure the fiery Test.