The Jargon File (version 4.4.7, 29 Dec 2003):
write-only language
 n.
    A language with syntax (or semantics) sufficiently dense and bizarre that
    any routine of significant size is automatically write-only code. A
    sobriquet applied occasionally to C and often to APL, though INTERCAL and
    TECO certainly deserve it more. See also Befunge.
The Free On-line Dictionary of Computing (30 December 2018):
write-only language
   A language with syntax (or semantics) sufficiently dense and
   bizarre that any routine of significant size is automatically
   write-only code.  A sobriquet applied occasionally to C and
   often to APL, though INTERCAL and TECO certainly deserve
   it more.