means adult in japanese. often used to describe anime pornography (it is NOT hentai. Hentai is a kanji (compound word) of the words Hen (strange, weird) and Tai (behavior, attitude, appearence etc etc) It is always used to describe a person and NEVER anime or manga) If someone was going to point out anime pornography, they would say "seijin-anime" or "seijin-manga". NOT "hentai"
average schmuck: hey man did u see that hentai NG is advertising?
smart person: its not hentai its seijin.
average schmuck: oh realy? I didnt know that