nil per os

This if for the woman in the hospital bed next to my mom. Mom didn't want to eat any dinner (other than a few bites of chocolate pudding) because she thought she might need to be "NPO." I asked for a term definition and she said "nil per...os - nothing by mouth. The woman behind the curtain said "oh no, it means nothing pre op." I looked it up and my mom is right. So there!