The Japanese slang H
Use of the slang "ecchi" in Japan
The Japanese pronounce the letter "H" as either "ecchi" (sounding similar to etchy) or "eichi". The former pronounciation is also used as a slang, either written phonetically in katakana or simply with the latin letter "H". In modern days, the casual slang, when used as an adverb or an adjective, roughly means sexual, or "sexy." When used as a noun, it could mean the act of sexual intercourse.
In fact, this is the most popular word chosen out of all the synonyms that mean "sex", by adults and by adolescents.
It could also be referring an act or an object that is sexually suggestive, or a person who likes sex too much.
The etymology of the slang "H"
There are three theories of etymology, two of which I think is plausible. First, I'll mention the one that I don't agree, and then the two more plausible explanations.
"Wada Heisuke" Theory
According to "Modern trendy words dictionary" (Jitsugyouei Nihon sha Publishing, 1933),
it is said that when the kanji to Mr. Wada Heisuke's name is reversed, it reads "sukebei tawa", and "sukebei," or "sukebe" is a slang for perv. However, whoever is Wada Heisuke is not explained, or why his name would be used for the slang.
Husband Theory
In a 1935 publication of a collection of secret code languages, published by the Osaka Police Association1, H is a slang derived from the initial of husband. This actually makes sense, since during the late Taisho era, female students are said to have used H to refer to lesbians.2
Hentai Theory
Well, less of a theory but more of a well-accepted fact that the slang H became widely known from a fictional series in Asahi Shinbun titled "shiroi magyo" (white devil fish)
during June 6, 1955 - March 15, 1956. In it, the author gives a character the nickname of H, the initial of Hentai.
Another earlier source also also supports this theory.
In the July 13th, 1952 issue of Asahi Weekly, in the article with a title that could be translated as "contemporary student vocabulary" it is mentioned that H is the head of "Hentai-sei" (perverted sexuality).4 Since this is a slang, it could well be the case that one person guessed the etymology, and another person bought the idea and encorporated it into a work of fiction.
Hentai can mean several things in biology and physics, however in a sexually related use, it is either an abbreviation of hentai seiyoku, or a person inclined that way. Hentai seiyoku is the degree of sexual desire or method of obtaining sexual satisfaction which is abnormal, or that goes against social norms.
3 Japanese love to abbreviate. Even
anime is an abbreviation.
H and hentai are used differently in Japan.
H has a more feminine connotation than, say, sukebe, which usually refers to male perverts (but can also refer to female pervs). Hentai, on the other hand, is a more crude word describing what is less socially acceptable. Note, this is a slang and is not a word the police would choose in categorizing various types of sex offenders.
The word hentai was popularly used in the abundant erotic literature that flourished around the late
Taisho era and early
Showa era
2.
With the introduction of Japanese
anime into the West, hentai is almost always used only when referring to the hentai anime genre.
Sources in Japanese
Most of the facts and sources came from a page (http://home.highway.ne.jp/sfuru/okigaru9803.html) written by Tsuru Furukawa.
Other sources:
1 - Reported in "Kindai Shomin Seikatsushi" (Nanpaku publishing)
2 - http://yam.vis.ne.jp/ka200101.htm
3 - http://dictionary.goo.ne.jp
4 - http://www.d1.dion.ne.jp/~kyoko_s/gogen2.html