Faut pas pousser mémé dans les orties can be literally translated by “Don’t push grandma in the nettles”. You can use this expression when you want to say to someone that he/she shouldn’t exaggerate that much! 

The expression dates back to the 20th century. It used to be “Faut pas pousser”. “mémé dans les orties” has been has been added later. And don’t you agree? It’s so much more funny!