The Collaborative International Dictionary of English v.0.48:

Brat \Brat\ (br[a^]t), n. [OE. bratt coarse garnment, AS. bratt
   cloak, fr. the Celtic; cf. W. brat clout, rag, Gael. brat
   cloak, apron, rag, Ir. brat cloak; properly then, a child's
   bib or clout; hence, a child.]
   1. A coarse garment or cloak; also, coarse clothing, in
      general. [Obs.] --Chaucer.
      [1913 Webster]

   2. A coarse kind of apron for keeping the clothes clean; a
      bib. [Prov. Eng. & Scot.] --Wright.
      [1913 Webster]

   3. A child; an offspring; -- formerly used in a good sense,
      but now usually in a contemptuous sense. "This brat is
      none of mine." --Shak. "A beggar's brat." --Swift.
      [1913 Webster]

            O Israel! O household of the Lord!
            O Abraham's brats! O brood of blessed seed!
                                                  --Gascoigne.
      [1913 Webster]

   4. The young of an animal. [Obs.] --L'Estrange.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Brat \Brat\, n. (Mining)
   A thin bed of coal mixed with pyrites or carbonate of lime.
   [1913 Webster]

WordNet (r) 3.0 (2006):

brat
    n 1: a very troublesome child [syn: terror, brat, little
         terror, holy terror]
    2: a small pork sausage [syn: bratwurst, brat]