Definition
An unhappy or disappointed person.
Origins
From Sexual Harassment Panda, an episode in 1999 of the animated comedy series South Park.
Opposite In Meaning
How People Actually Use It
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.
"I'm a saaaad panda, because my favorite band just broke up."
Source: Urban Dictionary