From 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.
From 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]