“legend having it a foolish king of the ancient Chinese Qin dynasty, upon seeing a deer, fatuously said ba instead of ka, and was the first to have earned himself the nickname baka” Japanese Street Slang, Peter Constatine pg 7
Why is baka (馬鹿- stupid) written with the kanji for horse plus deer?
March 31, 2008 at 1:37 pm (Japan and China, Japanese insults, Japanese language, Japanese slang, Japanese Street Slang- Peter Constatine, Japanese word origins, Kanji (Chinese symbols))
Japan Explained FASAQ 
BakaBaka said,
June 18, 2008 at 3:55 pm
YO! chinese here. The words that are written in kanji for baka, are also words found in the chinese language.(traditional chinese) It means horse and deer. Hmm japanese and chinese are quite alike. =D
alexcase said,
June 19, 2008 at 12:02 pm
So I guess the story must be true…
Ryoske said,
June 20, 2008 at 3:30 am
Actually, the Sino-Japanese reading of 鹿 would be ‘roku’ (‘ka’ is a kun reading). The Japanese Wikipedia article on ‘baka’ states 7 different theories on the word’s etymology, and according to this article, Japanese linguists consider the ‘馬鹿’ theory to be a folk etymology.
alexcase said,
June 21, 2008 at 11:02 am
Don’t think the Japanese pron necessarily disproves the theory, but translate a couple of other theories for us, pleeeeeease!
Ryoske said,
June 25, 2008 at 2:23 pm
The other theories according to http://ja.wikipedia.org/wiki/馬鹿#.E8.AA.9E.E6.BA.90
2. From a Sanskrit word used as slang by Buddhist monks. 莫迦, the other way of writing baka in kanji, is a transliteration of the Sanskrit ‘moha’ (ignorance, folly). The Kōjien dictionary adopted this theory and states that 馬鹿 is ateji (kanji used phonetically/semantically to represent native/borrowed words). There is actually a word ‘baka’ in Bengali (whose parent language is Sanskrit) which shares the meaning of ‘foolish person’ with Japanese. There are also doubts to this theory, however, as baka did not originally mean foolish person but ‘ruffian’ (The first written example of baka is in the 14th century Taiheiki as 馬鹿者 (bakamono), meaning ruffian).
3. The w in ‘wakamono’ (young person) was altered and became bakamono. Izuru Shinmura, editor of the Kojien, is said to have advocated this theory
4. From the attachment of the word 破家 (now another way of writing baka) appearing in Zen scriptures meaning ‘to become bankrupt’ with ‘mono’ to become 破家者 (person foolish enough to become bankrupt). This theory is adopted by the Nihon Kokugo Daijiten.
5. From a poem by Tang Dynasty Chinese poet Bai Juyi in which a wealthy Ma family (馬家) went into ruin after foolishly wasting their money.
6. From an alteration of the stem of ‘hakanai’ (fleeting, vain)
7. From a corruption of Old Japanese ‘woko’ (=oroka, foolish/stupid). There is also speculation that this is also the origin of ‘aho’, but without evidence
4 Ways To Use Baka (ばか) Like A Pro - Japanese Level Up said,
April 22, 2014 at 12:04 am
[…] way to call someone stupid in Japanese. The kanji has a lengthy history on why it is made up of horse-deer. And you will hear this word over and over in a million different contexts (hopefully none directed […]
Mark said,
September 15, 2014 at 1:49 am
When I was young I saw a deer near my house, and accidentally yelled “A HORSE!” when trying to get my family’s attention. They never let me live it down.
This made my night.
al said,
October 22, 2014 at 9:10 am
Because only idiots who will never comprehend proper written japanese make insignificant comparisons between kanji. You don’t ask yourself like: why is the word ‘idiot’ written with an ‘o’ after the ‘i’, do you?
Why not? Because you’d sound like a moron asking it. Don’t focus on that. You will never learn properly if you focus on things that doesn’t have any grammatical merit whatsoever.
alexcase said,
October 22, 2014 at 9:31 am
That’s complete nonsense. Unlike English letters, which have no meanings, the vast majority of kanji follow the meaning of the words they mean, and if you don’t notice that the kanji for kawaii or oishii don’t make sense in that way, you aren’t studying properly. It’s true that you don’t need to know the meanings and stories of those few like “baka” that don’t seem to make sense, but like learning English idioms, any old story will help you remember them. Also, stopping going through your life wondering “Why?” is kinda sad…
olog-hai said,
January 17, 2015 at 11:02 pm
Actually, English letters are derived from Phoenician/Hebrew letters, which are themselves pictograms with meanings (e.g. “A” is ultimately from “Aleph”, which means an ox; the Phoenician form is turned 90 degrees and one can see the horns of the ox).
alexcase said,
January 18, 2015 at 4:40 am
True and interesting, but I can’t really see how it’s relevant to this discussion.
Blinker said,
March 19, 2016 at 5:33 pm
There is no such word as Baka in Bangla(Bengali), however, there is a similar sounding word বোকা(transliteration:boka) which means stupid. My Bangla Academy dictionary lists the source as Sanskrit बुक्क(transliteration:bukka) which means goat. This makes a lot of sense because in Bangla, we often use the word for goat(ছাগল) and the word for ass/donkey(গাধা) to mean stupid or idiot which is used as an insult. Moreover, গাধা is generally considered more insulting than ছাগল.
Source: I am a native Bangla speaker, Bangla Academy.