癲
癲癇 tenkan epilepsy
瘋癲 fūten insanity; vagabond
癲狂 tenkyō madness

疒 is illness q.v., and / is upside-down q.v. acting as phonetic, and could have lent all sorts of meanings from being upside-down, like being upset or not normal. Someone who has an illness whereby one is upset and/or not normal is mentally unstable or crazy. Note that even in modern Japanese in compounds often has the meaning of being upset.

Mnemonic: You’re crazy if you’ve got an illness being not normal, upset, upside-down.

Verbose explanation and references

癲 is a combination of the signific 疒 disease (see below) and a character that here is acting as phonetic, / (see below). 顛 is the modern form. Note that even in modern Chinese / and 癲 stand for identically pronounced words or morphemes: diān. Further, as phonetic diān / seems to lend more than just its pronunciation. The original word diān carried the meaning ‘person upside-down’ (later more generally ‘overturn; topple; fall down; be overthrown’). Upside-down is in many languages a way to express being mentally unwell and Chinese is no exception. Confusingly, / is also used for a different word with the meaning ‘top; summit’.

疒

疒 is Kangxi radical 104 sickness/illness. The left part originally depicted a bed while the remaining part according to Henshall (381) used to be a variant of person 人 (a person on bed, conveying being ill). Henshall writes that the left side of 疒 was derived from the left side of tree 木 (half a tree) thereby conveying a plank. In the seal character shape (see the picture at the right) 疒 looks more like a real bed, with legs, seen from the side, turned 90 degrees (but in that variant 人 seems absent). In its modern shape 疒 seems to have more in common with 广, which originally depicted a large building.

/ consists of 眞/真 (on itself now used for an unrelated word) and head[1]. 眞/真 can be further analysed as inverted head 県 (from 首) and fallen person ヒ, expressing person upside-down (leading to more modern meanings like overturn). The extra head 頁 was added to separate / from the unrelated word 眞/真 had gotten used for. As often the seal characters (see the right) give a better idea of the pictographic nature of the elements (頁 being an exaggerated head with legs bungling underneath, 首 having hairs on top, hence the hairs on the bottom-side of the inverted head 県, etc. See Henshall 93, 139, 273, 514 for more on the full history of these elements; see also Wieger 10 L and 160 A).

1. In Japanese 頁 is now used for the word peeji ‘page’ but in characters like 顔 kao ‘face’ its original meaning can still be discerned.
2. About the vocubulary, 瘋癲 fūten is not very common, but according to Twitter-frequencies (one or two tweets a day) not unheared of. Also, it’s used in the book title of 谷崎 潤一郎 Tanizaki Jun’ichirō’s 『瘋癲老人日記』 Fūten rōjin nikki, Diary of a mad old man. As for 癲狂 tenkyō, that’s pretty rare, but still a tweet every other day. Even the archaic word 癲狂院 tenkyōin ‘asylum’ comes by now and then. 癲癇 tenkan on the other hand, being a serious medical term, is very relevant (though for public consumption it might get simplified to てんかん, as in てんかん発作 tenkan hossa ‘epileptic seizure’).
3. As noted in the text, 顛 used to stand for two different words, one meaning ‘top; summit’ and the other ‘overturn’ (and this lead to different words in Japanese as well). However, it seems to me that in Classical Chinese the word ‘overturn’ had connotations from its older meaning of person upside-down, like being upset. Even in modern Japanese words there seem to be signs of that. For example in 顛倒 tentō, the primary meanings are ‘falling down; reversing’ but there’s also the meaning of ‘getting upset’. Further, pronounced tendō, it’s a Buddhist term for ‘cognitive distortion’. In 動顛 dōten even the primary meaning is ‘being upset; being surprised and stunned’ (according to Edict). Seems to me proof that used as phonetic, had a substantial connotation of a disturbed mental state.
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