Source : Webster's Revised Unabridged Dictionary (1913)

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

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

Source : WordNet®

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: {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}]
     3: thick soft fur lying beneath the longer and coarser guard
        hair [syn: {underfur}]
     v : cover with a primer; apply a primer to [syn: {prime}, {ground}]