I was eighteen before I heard anyone use any variety of Jewish as an insult. I grew up in a predominantly Jewish area, so the idea that anyone would ever do such a thing was completely foreign to me. When I went away to college a woman told me her computer was being Jewish, ie cheap. (Yea, I still feel the need to translate this because it's still completely foreign to me.) I could tell it wasn't a compliment but I had to ask her what she meant. She was immediately embarrassed, but once she fessed up, I explained to her why it wasn't okay, and that I didn't care what she said but she wasn't going to say ish like that to me, not in my room.
Made me really popular, naturally. Whatever, I transferred away from there as quickly as I could.
Sent from my phone, please excuse typos or brevity.