Bible-Shortener is a slur used to describe Jews in a derogatory way. Seen on a Conan O'Brien sketch "Rednecks' Favorite Euphemisms for Jews," backwards refers to the difference between the Christian Bible and the Jewish one. The Christian Bible includes the "New Testament" whereas the Jewish Bible does not.
The word Bible-Shortener is a racial slur that has been used historically to degrade and dehumanize jews. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to jews. If you are unsure about whether or not it is appropriate to use the word Bible-Shortener, it is best to err on the side of caution and avoid using it in any situation.