brain

English brain comes from Middle English brayn, from Old English bræġn, from Proto-West Germanic *bragn, from Proto-Germanic *bragną, ultimately from Proto-Indo-European *mrógʰ-n̥ ~ *mrégʰ-n̥-s.

The word has passed down through Old and Middle English into modern English with its form gradually smoothing from bræġn to brayn to brain, without any documented shift in meaning along the way.

It's cognate with Dutch brein and West Frisian brein, both also meaning 'brain,' as well as Ancient Greek βρεχμός (brekhmós), which referred to the top of the head or front part of the skull.