"treatment of disease by the external application of metals," by 1877, from metallon, Greek source of metal (n.) + therapy.
First formulated as a system by Burq in 1848, and hence often called Burqism, it has been recently revived by Charcot. Simple disks of various metals are employed in contact with the external parts of the body, from which different therapeutic results are claimed. Other observers assert that all the phenomena described as following the application of metals may be produced by disks of wood, and that whatever curative results are attained are due to mental effects, rather than to any special virtues emanating from the metals themselves. [Century Dictionary, 1895]