Found one definition

  1.                 From GNU Collaborative International Dictionary of English, version 0.54:
                    

    Undercoat (?), 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]