What does エッチ Ecchi mean?

Ecchi is the letter H, said aloud in Japanese. The H is the first letter of hentai written in the Roman alphabet, so the word is an abbreviation standing in for a blunter one, in the way English says the F word.

The mechanics of the euphemism

変態 (hentai) means, in plain use, an abnormality or a perversion, and is a real word with clinical and everyday senses. Romanised, it begins with H. Japanese names that letter roughly as ecchi, and speakers began using the letter in place of the word.

Euphemism by initial is an ordinary move across languages, and it works the same way here: the shortened form is softer, more sayable, and slightly arch, precisely because everyone knows what it is standing in for.

What it does in a sentence

As an adjective it means lewd, dirty or naughty, and is often teasing rather than accusatory. As a noun it labels someone as a lech. In a common verb phrase it means to have sex, which is the most everyday of its uses and the one most likely to surprise an English speaker who has only met the fandom sense.

The register is mild and euphemistic. It is the word available when the direct one would be coarse, which is a very different social position from the one English gives it.

The English tier system

English-speaking fandom sorted these two words into a scale: hentai for explicit material, ecchi for suggestive material that stops short. That distinction is genuinely useful, widely understood among English speakers, and entirely an English construction.

It cannot be read back into Japanese, where the pair are not two points on a scale but one word and its abbreviation. The English scale is a real thing that English fandom built and then, understandably, assumed it had imported.

Why the invention happened

Because English needed the distinction and had no vocabulary for it. Categorising material by how explicit it is matters commercially and socially, and English had only clinical terms or crude ones, neither usable in ordinary conversation about a television programme.

Two Japanese words were available and looked like a pair. Assigning them to two rungs solved a real problem, and the solution has outlived any awareness of where the words came from.

The accurate summary

In Japanese: one word, its initial, a euphemism, in daily use, fairly mild. In English: a two-tier classification system that Japanese does not have. Both are functioning descriptions of how their own speakers use the terms, and only one of them is about Japanese.