The Jargon File (version 4.4.7, 29 Dec 2003):

screaming tty
 n.

    [Unix] A terminal line which spews an infinite number of random characters
    at the operating system. This can happen if the terminal is either
    disconnected or connected to a powered-off terminal but still enabled for
    login; misconfiguration, misimplementation, or simple bad luck can start
    such a terminal screaming. A screaming tty or two can seriously degrade the
    performance of a vanilla Unix system; the arriving ?characters? are treated
    as userid/password pairs and tested as such. The Unix password encryption
    algorithm is designed to be computationally intensive in order to foil
    brute-force crack attacks, so although none of the logins succeeds; the
    overhead of rejecting them all can be substantial.


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

screaming tty

   [Unix] A terminal line which spews an infinite number of
   random characters at the operating system.  This can happen if
   the terminal is either disconnected or connected to a
   powered-off terminal but still enabled for login;
   misconfiguration, misimplementation, or simple bad luck can
   start such a terminal screaming.  A screaming tty or two can
   seriously degrade the performance of a vanilla Unix system;
   the arriving "characters" are treated as userid/password pairs
   and tested as such.  The Unix password encryption algorithm is
   designed to be computationally intensive in order to foil
   brute-force crack attacks, so although none of the logins
   succeeds; the overhead of rejecting them all can be
   substantial.

   [Jargon File]