courage; spirit; personal substance; "guts"
A leader cannot just be a man of good ideas; he must also be a man of mettle, who fights to win.
metallon (mine, quarry, pit, metal, ore) (Greek)
Sounds like metal and metals are obviously strong. So, can be related to a strong courageous person. Reminds of Steel King industrialist Lakshmi Mittal: Symbolic of courage to carry on