Why is the Brain called Brain in English?

What is the etymology of the word brain in English?

A no-brainer this question?

Nope.

A real puzzle.

If you are interested in the matter take a look, for example, at the Oxford University Press OUP Academic insights into thinking world

Anatoly Liberman gives some food for thought about the word brain.

A real brainer, I say!

Let me give you a taste from that fabulous blog:

"One of the ideas present in Indo-European religion was that the gods and people used different names for the same objects. According to a Scandinavian myth, the world was made from the body parts and organs of the dismembered giant Ymir. His brain, called heili in Old Icelandic, became the sky. The origin of heili is unknown (this is not a surprise), but, characteristically, it is a different word from hjarni, the noun that 19th-century linguists compared with Gothic hwairnei. Heili was the designation of a primordial brain and perhaps aroused loftier associations; hjarni filled the skulls of mortals. (It gives me great pleasure to report that Modern Icelanders call the human brain heili and thus elevated our pulp to divine dimensions.) A leading modern etymologist thought that hjarni might be a cognate of the Icelandic word for “gray” and glossed hjarni as “gray matter.” I think he was mistaken, but he may have looked for an answer in the right direction. Hjarni, like German Hirn “brain” (a more common word is Gehirn), is more probably related to German Harn “urine”, whose original meaning was “bodily waste.” Such is my uncomplimentary picture of the human brain seen through the eyes of our ancestors."
Anatoly Liberman
Why is the Brain Called a Brain, Or, the Questionable Value of Grey Matter