Although megillah is a slang word in English, it has perfectly respectable Hebrew origins. Megillah derives from the Yiddish megile, which itself comes from the Hebrew word mĕgillāh, meaning "scroll" or "volume." (Mĕgillāh is especially likely to be used in reference to the Book of Esther, which is read aloud at Purim celebrations.) It makes sense, then, that when megillah first appeared in English in the early 20th century, it referred to a story that was so long (and often tedious or complicated) that it was reminiscent of the length of the mĕgillāh scrolls. The Hebrew word is serious, but the Yiddish megile can be somewhat playful, and our megillah has also inherited that lightheartedness.