A phrase that people use when they want to be considered "fashionably late" but are actually on time for the event in question. People who say they are "fashionably on time" are usually either joking or have an inflated sense of self-importance.
Guest 1: I'm here; I guess I'm fashionably late.
Guest 2: Actually, you're not late. The party is starting right now.
Guest 1: Oh, well, in that case...I must be...fashionably on time!