Unreadable or indecipherable. Something of sufficient complexity that it is incomprehensible. Often applied to someone else's style of writing software.
I've spent an hour trying to figure out what your freaking write-only PERL script does: lean to use some comments for a change!

1. <jargon, humour> (WOM) The obvious antonym to "{read-only memory}" (ROM). Out of frustration with the long and seemingly useless chain of approvals required of component specifications, during which no actual checking seemed to occur, an engineer at Signetics once created a specification for a write-only memory and included it with a bunch of other specifications to be approved. This inclusion came to the attention of Signetics management only when regular customers started calling and asking for pricing information. Signetics published a corrected edition of the data book and requested the return of the "erroneous" ones. Later, around 1974, Signetics bought a double-page spread in "Electronics" magazine's April issue and used the spec as an April Fools' Day joke. Instead of the more conventional characteristic curves, the 25120 "fully encoded, 9046 x N, Random Access, write-only-memory" data sheet included diagrams of "bit capacity vs. Temp.", "Iff vs. Vff", "Number of pins remaining vs. number of socket insertions", and "AQL vs. selling price". The 25120 required a 6.3 VAC VFF supply, a +10V VCC, and VDD of 0V, +/- 2%. 2. bit bucket. [Jargon File] (2007-03-24)

[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]