The Collaborative International Dictionary of English v.0.48:

Dreg \Dreg\ (dr[e^]g), n. [Prob. from Icel. dregg; akin to Sw.
   dr[aum]gg, cf. Icel. & Sw. draga to draw. Cf. Draw.]
   Corrupt or defiling matter contained in a liquid, or
   precipitated from it; refuse; feculence; lees; grounds;
   sediment; hence, the vilest and most worthless part of
   anything; as, the dregs of society.
   [1913 Webster]

         We, the dregs and rubbish of mankind.    --Dryden.
   [1913 Webster]

   Note: Used formerly (rarely) in the singular, as by Spenser
         and Shakespeare, but now chiefly in the plural.
         [1913 Webster]

WordNet (r) 3.0 (2006):

dreg
    n 1: a small amount of residue