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Ecchi (or Boku no Nuqman, from the Japanese エッチ etchi) is from the Japanese word meaning "lewd", "sexy", "pervert", or "naughty" when used as an adjective, or it can mean to have sex when it is used as a verb or noun. The English use of this word is usually used in manga and anime, used when it has sexual content, such as not much clothing or nudity, but does not show sex.
Ecchi anime is also sometimes filled with innuendo and sexual humor. Ecchi anime doesn’t show sex, at least not in a clear way.
Ecchi is similar to hentai, though it doesn’t go as far. The term itself may be from the word hentai, but the origins are unclear. Ecchi means “h”.