To be anencephalous is to be brainless, empty-headed, to have a skull with an echo.
It is always more polite to use the medical term for an offensive idea. Here is the medical term for "brainless." It has the added benefit that those you would use it for will not understand it! It offers a wonderful way to vent your frustration inoffensively while building your vocabulary. What a bargain! It comes from the noun anencephaly "the absence of a brain."
We are all constantly bumping into people who seem to have nothing in their heads, many of whom drift into positions of considerable power: "I don't know how many anencephalous presidents this company can endure and stay solvent." We may safely extend the meaning of this word to the product of the witless: "Have you read the latest anencephalous memo from headquarters, the one about saving electricity by not using electric pencil sharpeners?"
This funny word is a Latinization of Greek anenkephalos, used by the great 2nd century Greek physician Galen. It is made up of four elements, a(n)- "without" + en "in" + kephale "head" + an adjective suffix. We find the prefix a(n)- in many borrowed English words like amoral, amorphous (without shape), and agnostic. The N is inserted before vowels, as it is when using the English article a(n), as an apple. En comes from the same source as English in. Greek kephale "head" comes from the same Proto-Indo-European word as English gable and German Giebel "gable."