In the context of the
doujinshi community, a subset of the anime & manga community,
Sad Panda refers to a certain hidden section of the site e-hentai.org. E-hentai provides a variety of services, most importantly is its gallery, holding gigantic collections of Japanese doujinshi, Western fanfic, image collections, etc. While e-hentai hosts a public gallery where everyone can view content, it also has a parallel gallery that is exclusive to registered members. This parallel gallery is dubbed Sad Panda, because if one tries to access the site without proper credentials, the image of a
sad panda is displayed. The reason people use such indirect reference to the site is because (in case word
hentai haven't spelt it out) most content on the site is NSFW and many FB groups or public forums ban links to NSFW sites. People can share NSFW links by giving
the gallery ID and mentioning the source is sad panda, although the latter is often omitted because the gallery
ID's format is easy to spot.