"dirt, dregs," Old English dros "the scum thrown off from metals in smelting," from Proto-Germanic *drohs- (cognates: Middle Dutch droes, Dutch droesem, Middle Low German dros, Old High German truosana, German Drusen "dregs, husks"), from PIE dher- (1) "to make muddy." Meaning "refuse, rubbish" is mid-15c.