Don't get this tattoo. More on reddit.com

Cerebrum secundum. This is neat because while it totally means "second brain" it also has the added wordplay of meaning "favorable brain", as the adjective "secundus" means both "second" and "favorable". Take that as you will. More on reddit.com

A quick search tells me that cerebellum ... of the brain you have referenced. It was composed of cerebrum and the diminutive ending -ellum. Let's call in the big guns. @Cinefactus Cinefacte, habesne sententiam? Oremus pro Pontifice nostro Francisco Vastatore: fiant dies ejus pauci. If you ever catch me posting something in Latin, of your charity, please check and correct it. ... It is indubitable that the word cerebellum ... More on latindiscussion.org

Is this just a crazy coincidence, Yes. They are both relatively short words, and it's not particularly strange for two unrelated languages to happen to have words for the same concept start with the letter /n/ and have a close-mid back rounded vowel. Additionally, Japanese borrowed this reading of 腦 from Middle Chinese (in which it is pronounced /nau/, already further from the Greek than the Japanese), which was spoken roughly from 400-1100 AD, whereas Ancient Greek was spoken from around 1000-300 BC. So there isn't even overlap. In Old Chinese, which at least overlaps with Ancient Greek in terms of when it was spoken, it's reconstructed as /*nˁ[u]ʔ/ or /*nuːʔ/ or /*nɑuʔ/, depending on the linguist. Furthermore, I think you misrepresent how similar these words' meanings are. As far as I'm aware, 腦 in Chinese has always referred primarily to the physical organ, with uses akin to "mind" being metaphorical. After all, it uses the meat radical 肉. The Greek word νόος (nó.os), by contrast, does not ever refer to the organ as far as I'm aware and refers solely to the mind as the seat of rational thought and feeling. I'm less knowledgeable about Ancient Greek, but it does have the completely different word ἐγκέφᾰλος (enképhalos) for the body part, from which we get "encephalitis" and other such medical gems. While it's definitely possible for words' meanings to shift over time, it's also common for shoddy linguists to use superficially similar words that refer to vaguely related concepts as evidence of some pretty poorly supported theories. And honestly the most convincing evidence is non-linguistic -- there's no evidence that the Ancient Greeks and Chinese ever had contact with one another, and that would make it pretty much impossible for these words to be related. More on reddit.com