dross (n.)

"dirt, dregs, refuse or impure matter which separates from a liquid and falls to the bottom or rises to the top," Old English dros "the scum thrown off from metals in smelting," from Proto-Germanic *drohs- (source also of Middle Dutch droes, Dutch droesem, Middle Low German dros, Old High German truosana, German Drusen "dregs, husks"), perhaps from a PIE dher- "to make muddy." Figurative sense of "worthless remainder of what was once valued" is from late 14c. Meaning "refuse, rubbish" is mid-15c.