Write-only memory is, of course, the opposite of read-only memory.
From the Jargon File:
The obvious antonym to read-only memory. 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, in 1972, 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%.