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.

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.

An unhappy, disappointed person. The phrase can be traced back to an episode of the cartoon South Park, in which the "Sexual Harassment Panda" teaches the children what is and isn't sexual harassment.

For extra sadness, the "a" in sad is drawn out.

A sexual position involving fellatio, makeup and crying.

The saddest of all animals is the panda. He has no friends, no family, and no future. Therefore, a sad panda is the saddest of the sad. This term is used as an adjective or exclamation to describe moments of sadness beyond sadness but the utter depths of existential angst and isolation.

A melancholy-centered bear who has extreme feelings for a bipolarbear.

Hey, I'm a guy. I'm single. I would never cheat on you. I would always text you first, tell you that you're beautiful every day, say I love you in front of my friends, never choose my video games over you, protect you, try my best to never make you cry, hold you and never let you go, kiss you in the pouring rain. Yes, ladies, I do exist. No, I'm not gay. Stop dating douche-bags and jerks. Date me.