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

debugging by printf

    The debugging technique where the programmer
   inserts print statements into a program so that when run the
   program leaves a "trail of breadcrumbs" allowing him to see
   which parts were executed.  The information output may just be
   a short string to indicate that a particular point in the code
   has been reached or it might be a complete stack trace.  The
   output typically just goes to the window or terminal in which
   the program is running or may be written to a log file.

   printf is the standard C print function, other languages
   would use different names.

   (2007-03-08)