The Jargon File (version 4.4.7, 29 Dec 2003):
write-only code
 n.
    [a play on read-only memory] Code so arcane, complex, or ill-structured
    that it cannot be modified or even comprehended by anyone but its author,
    and possibly not even by him/her. A Bad Thing.
The Free On-line Dictionary of Computing (30 December 2018):
write-only code
   [a play on "read-only memory"] Code so arcane, complex, or
   ill-structured that it cannot be modified or even comprehended
   by anyone but its author, and possibly not even by him/her.  A
   Bad Thing.
   [Jargon File]