The Collaborative International Dictionary of English v.0.48:

copybook \copybook\ n.
   a book containing models of good penmanship; used in teaching
   penmanship.
   [WordNet 1.5] copycat

WordNet (r) 3.0 (2006):

copybook
    n 1: a book containing models of good penmanship; used in
         teaching penmanship

The Free On-line Dictionary of Computing (30 December 2018):

copybook
copy member
copy module

    (Or "copy member", "copy module") A
   common piece of source code designed to be copied into many
   source programs, used mainly in IBM DOS mainframe
   programming.

   In mainframe DOS (DOS/VS, DOS/VSE, etc.), the copybook
   was stored as a "book" in a source library.  A library was
   comprised of "books", prefixed with a letter designating the
   language, e.g., A.name for Assembler, C.name for Cobol, etc.,
   because DOS didn't support multiple libraries, private
   libraries, or anything.  This term is commonly used by COBOL
   programmers but is supported by most mainframe languages.
   The IBM OS series did not use the term "copybook", instead
   it referred to such files as "libraries" implemented as
   "partitioned data sets" or PDS.

   Copybooks are functionally equivalent to C and C++
   include files.

   (1997-07-31)