Pronounced meh-GILL-eh, to rhyme with "guerilla." Hebrew: "scroll"
1. Megillah usually describes the book of Esther, which is read in the synagogue during the Purim holiday; also the Book of Ruth (There are five megillahs in all.)
2. Anything very long, prolix; a rigmarole. The Book of Esther wanders through a crushing concentration of detail, and the devout sit through the long, long reading after a day of fasting.
3. In popular parlance: Anything complicated, boring, overly extended, fouled up. "He'll put you to sleep with that megillah." "Don't give me a megillah" means "Spare me the full, dull details."
(from Leo Rosten's "The Joys of Yiddish"--there are more examples and a joke in his book, if anyone's interested, let me know and I'll add those here)
Pur, Hebrew for "lot", hence the word "Purim", for casting lots.