Webster's English Dictionary

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brain

1. brain \'bra-n\ n [ME, fr. OE brgen; akin to MLG bregen brain, Gk 
   brechmo]s front part of the head 1a: the portion of the vertebrate central 
   nervous system that constitutes t he organ of thought and neural 
   coordination, includes all the higher nervous centers receiving stimuli 
   from the sense organs and interpreting and correlating them to formulate 
   the motor impulses, is made up of neurons and supporting and nutritive 
   structures, is enclosed within the skull, and is continuous with the spinal 
   cord through the foramen magnum 1b: a nervous center in invertebrates 
   comparable in position and function t o the vertebrate brain 2a: INTELLECT, 
   INTELLIGENCE - often used in pl.  2b1: a very intelligent or intellectual 
   person  2b2: the chief planner of an organization or enterprise - usu. used 
   in pl .
2. brain vt 1: to kill by smashing the skull  2: to hit on the head