The disease, Down's Syndrome--el síndrome de Down--was once called "Mongoloid idiocy," idiotez mongoloide. So it is a play on words, a combination of "Bong" (the pipe used for smoking cannabis) with "mongoloid." It implies someone whose brain has been damaged by excessive marijuana smoking.
The doctor who described the disease, Down, thought that the faces of children with the disease resembled Mongolians.
Obviously it is a slang word, and a VERY impolite one. It is disrespectful to call anyone an idiot; the old term "Mongoloid idiot" is disrespectful both to Mongolians and to people with Down's Syndrome.
It is pretty bad when you have a word that insults three different groups at the same time!