"dirt, dregs," Old English dros "the scum thrown off from metals in smelting," from Proto-Germanic *drohs- (cf. 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.
the impurities of silver separated from the one in the process of melting (Prov. 25:4; 26:23; Ps. 119:119). It is also used to denote the base metal itself, probably before it is smelted, in Isa. 1:22, 25.