The Collaborative International Dictionary of English v.0.48:

Undercoat \Un"der*coat`\, n.
   1. A coat worn under another; a light coat, as distinguished
      from an overcoat, or a greatcoat.
      [1913 Webster]

   2. A growth of short hair or fur partially concealed by a
      longer growth; as, a dog's undercoat.
      [1913 Webster]

WordNet (r) 3.0 (2006):

undercoat
    n 1: seal consisting of a coating of a tar or rubberlike
         material on the underside of a motor vehicle to retard
         corrosion [syn: undercoat, underseal]
    2: the first or preliminary coat of paint or size applied to a
       surface [syn: flat coat, ground, primer, priming,
       primer coat, priming coat, undercoat]
    3: thick soft fur lying beneath the longer and coarser guard
       hair [syn: undercoat, underfur]
    v 1: cover with a primer; apply a primer to [syn: prime,
         ground, undercoat]