The Collaborative International Dictionary of English v.0.48:

Cod liver \Cod" liv`er\, n.
   The liver of the common cod and allied species.
   [1913 Webster]

   Cod-liver oil, an oil obtained from the liver of the
      codfish, and used extensively in medicine as a means of
      supplying the body with fat in cases of malnutrition.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cod-liver oil
    n 1: an oil obtained from the livers of cod and similar fishes;
         taken orally as a source of vitamins A and D [syn: cod-
         liver oil, cod liver oil]