Whether it’s a long-winded tale, or a story overloaded with details--it’s called a “whole megillah!”(In “the old country” they would have said “a gantse megillah!”)
So what exactly is a “megillah”?
Technically, a megillah is a rolled scroll. Specifically, the term megillah is used to describe the five canonical works from the Ketuvim (Writings) section of the Bible that are read in the synagogue on different holidays. The five megillot are:
Shir HaShirim - The Song of Songs - written by King Solomon and read on Passover.
Ruth - The Book of Ruth - written by Samuel and read on Shavuot.
Eichah - Lamentations - written by Jeremiah and read on Tisha B'Av.
Kohelet - Ecclesiastes - also written by King Solomon and read on Sukkot.
Esther - The Book of Esther - written by Mordechai and Esther and read on Purim.
When preceded with a definite article, however, “the Megillah,” refers specifically to the Book of Esther. Megillat Esther is the only one of the five megillot which one is obligated to read/hear. In fact, on Purim, one should hear it read both at night and during the day.
As for the catchy phrase “the whole megillah”--according to WorldWideWords.org, it came into the English vernacular in a variety of forms through its use by Jewish entertainers. The specific wording of “the whole megillah,” however, had its first recorded colloquial usage on Rowan & Martin’s Laugh-In in 1971.