Hello! Another intricate question! The word ‘brain’ comes from Old English Language (Anglo-Saxon so probably before 1066 and the Battle of Hastings) and was probably derived from a variation of a word used to describe the inside of the head – this word was probably something like ‘bragen’. I think the word ‘brain’ was first used around the year 1800.
I’ll be honest-had to look this up. ‘The New Oxford Dictionary of English (1998) defines brain as “an organ of soft nervous tissue contained in the skull of vertebrates, functioning as the coordinating centre of sensation and intellectual and nervous activity”. As early as 500 BC Alkmaion of Kroton, based on anatomical evidence, proposed that the brain was essential for perception, (Doty, 2007).” I took this from http://www.kopfinstruments.com/Carrier/downloads/Carrier65.pdf