What is "no-brainer"

Brain \Brain\ (br[=a]n), n. [OE. brain, brein, AS. bragen, br[ae]gen; akin to LG. br["a]gen, bregen, D. brein, and perh. to Gr. bre`gma, brechmo`s, the upper part of head, if [beta] = [phi]. [root]95.]

1. (Anat.) The whitish mass of soft matter (the center of the nervous system, and the seat of consciousness and volition) which is inclosed in the cartilaginous or bony cranium of vertebrate animals. It is simply the anterior termination of the spinal cord, and is developed from three embryonic vesicles, whose cavities are connected with the central canal of the cord; the cavities of the vesicles become the central cavities, or ventricles, and the walls thicken unequally and become the three segments, the fore-, mid-, and hind-brain.

   Note: In the brain of man the cerebral lobes, or largest part of the forebrain, are enormously developed so as to overhang the cerebellum, the great lobe of the hindbrain, and completely cover the lobes of the midbrain. The surface of the cerebrum is divided into irregular ridges, or convolutions, separated by grooves (the so-called fissures and sulci), and the two hemispheres are connected at the bottom of the longitudinal fissure by a great transverse band of nervous matter, the corpus callosum, while the two halves of the cerebellum are connected on the under side of the brain by the bridge, or pons Varolii.

2. (Zo["o]l.) The anterior or cephalic ganglion in insects and other invertebrates.

3. The organ or seat of intellect; hence, the understanding; as, use your brains. `` My brain is too dull.''
   --Sir W. Scott.

   Note: In this sense, often used in the plural.

4. The affections; fancy; imagination. [R.]
   --Shak.

5. a very intelligent person. [informal]

6. the controlling electronic mechanism for a robot, guided missile, computer, or other device exhibiting some degree of self-regulation. [informal]

   To have on the brain, to have constantly in one's thoughts, as a sort of monomania. [Low]

   no-brainer a decision requiring little or no thought; an obvious choice. [slang]

   Brain box or Brain case, the bony or cartilaginous case inclosing the brain.

   Brain coral, Brain stone coral (Zo["o]l), a massive reef-building coral having the surface covered by ridges separated by furrows so as to resemble somewhat the surface of the brain, esp. such corals of the genera M[ae]andrina and Diploria.

   Brain fag (Med.), brain weariness. See Cerebropathy.

   Brain fever (Med.), fever in which the brain is specially affected; any acute cerebral affection attended by fever.

   Brain sand, calcareous matter found in the pineal gland.

alt. (context informal English) An easy or obvious conclusion, decision, solution, task, etc.; something requiring little or no thought. n. (context informal English) An easy or obvious conclusion, decision, solution, task, etc.; something requiring little or no thought.

n. anything that requires little thought