This story is several years old and takes place at a time when my grandmother was still working.
She was part of the department that was there to get reviews and comments on the health care of the hospital she was working for. (Funnily enough, she was submitted to that hospital a few years later for an operation and my mother had some polite, but decided words about the health care my grandmother was receiving).
Somehow, my grandmother was on the phone with this girl (well, ‘girl’ meaning probably in her 20s or older, but young enough to be a girl to my grandmother) and she was beginning to be annoying. Somehow, she started a discussion on what weight a baby should have when it is X months/years old. Then she pulled the I Am Important card.
“I should know, after all,” she said, proud as punch. “I am a Registered Nurse.”
My grandmother replied, in her nicest deadpan-snarker voice, “I am sure your mother is very proud of you.”
And that is how that line has become our go-to way of being the politest way to tell someone to fuck off.