from what I remember, PA used that name first, I've not read it here as a derogatory name for her (but I've not read every post so might be wrong but in general I don't think she has been referred here to that name before PA said it)...maybe someone who has more info might be able to clarify.
I didn't realise until you pointed it out Cazz that it was Peter who had called her that. The first I heard about it was when someone called her it on this thread, which shocked me. I read back through the thread where it was first discussed that he had called Emily "Embolism" and a lot of posters also found it distasteful. I hope it doesn't become her nickname on here