Green’s Dictionary of Slang

a long, tedious or complicated story, a complicated state of affairs, a long explanation; esp. in phr. the whole megillah, everything, the lot.

1943		D.O. Selznick Memo 10 July in Behlmer Memo from D. O. Selznick (1972) 334: Sydney Guilaroff [...] was in wrong with the union because of an elaborate megillah.
1954		B. Wolfe Late Risers 134: Frana slipped her coat off and began to remove her sweaters. ‘Oh, come on, Frana. Not the whole megilleh.’.
1958		Mad mag. Jan.–Feb. 48: Big eyes should be made of more solid megillah.
1963		in T.I. Rubin Sweet Daddy 126: Everything we make such a big migila.
1971		T. Thackrey Thief 118: And that was it. The whole megillah.
1976		New Yorker 16 Aug. 26: She uses expressions like ‘the whole megillah’ (meaning the whole long story) and ‘humongous’ (meaning huger than huge and more tremendous than tremendous).
1983		E. Litvinoff Falls the Shadow 59: I could give you the whole megilla – I’ve got pretty good recall – but it would take too long.
1992		L. Sanders McNally’s Luck 279: Listen, let’s go through the whole megillah one more time from the top.
2002		J. Barth Coming Soon!!! 321: Anyhow, the whole megillah will no doubt come out in the official inquiry.
2021		J. Ellroy Widespread Panic 280: ‘I see Bill Parker behind the whole magillah’.