Joshing You: Came from the Jewish name "Joshua" and is used to imply that you are dishonest, a cheat, or misleading. Meaning Jewish people are dishonest.
When I was in grade school many years ago, a kid got in trouble for saying "I was just joshing you". Her explanation was that it was derogatory to Jewish people from using the Jewish name "Joshua" to imply that you got cheated or misled, etc. I gathered that it meant jewish people would cheat you, lie to you, or take back what they said, and it became known as "Joshin". My teacher could have been wrong, but it made sense to me.