Yonie is one of those terms that can be nice or derogatory. It depends on who is saying it and the context. I believe the word originates as a Pennsylvania Dutch term meaning “Johnny”. You Dutchies can clear that up if you want.
I grew up understanding it as a term for Amish. Many people (even Mennonites) have not heard of the term. I finally found some backing: the UrbanDictionary lists it as slang for Amish.
I always took the meaning farther and used it to describe conservative Mennonites as well. If I’m being derogatory, I will use it to describe anyone who is more conservative than me.
Here’s the catch. Mennonites and Amish are permitted to use it (read the heritage blog). No non Mennos and Amish are not. There is no positive way for them to use it. It is similar to a certain word that African Americans are permitted to use but close to illegal for a white person to use.
If you are a Mennonite and you get called a Yonie, turn the other cheek. You are not permitted to strike back or show violence according to your faith. Go to all of your friends and tell them what that horrible person called you. That is the Mennonite way.