"Hentaionline" is a Japanese term, and it consists of two words: "hentai" and "online." The word "hentai" (変態) refers to perverted or unusual sexual desires in general. In the context of adult content, hentai typically means animated pornography, specifically anime porn.
"Online" is a self-explanatory term that refers to something available on the internet. When combined with "hentai," it implies that the content being referred to can be found and accessed online. In other words, "hentaionline" refers to hentai or adult animated pornography that is accessible via the internet.
So, a tag like "hentaionline" would indicate an adult video featuring animated pornography that can be viewed on the web. This content is meant for a mature audience and viewers who are familiar with and interested in this specific category of adult animation.