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

Call Unix

    (cu) The original Unix virtual
   terminal utility.  cu allows a user on one computer to log in
   to another connected via Ethernet, direct serial line or
   modem.  It shares some configuration files with UUCP in
   order to be able to use the same connections without conflict.

   Unix manual page: cu(1).

   (1997-12-01)