A fusion of the two words hentai and Thao.
It refers to the act of making sweet love to an Asian (preferably Vietnamese) person, who is simultaneously trying to both pleasure and beat you up. Normally this includes a lot of punches to the abdominal region.
George: Dude did you get to have sex with that hot Vietnamese last night?
Michael: Yep! But it turned out (S)he only wanted to do hen-thao...