Pretty much it's been called that at United for a long time. I fly Fifi. Doesn't bother me a bit when someone calls it that. Hell, the plane calls you a retard a minimum of twice every leg. If someone is truly bother by anothers nickname for their airliner they are letting the wrong s--t get under their skin. My opine of course.

And of course the LUAL guys know the LCAL guys are very sensitive to the name Guppy, which is part of why they do it (it's also part of the UAL corporate culture). And pointing out on a public forum that you are an LCAL guy and sensitive to it is NOT going to make it stop any sooner.