The mysterious, misunderstood threat that propels the classic film noir Kiss Me Deadly; it turns out to be a suitcase of a glowing, uncontrollable radioactive substance.
The great whatsit need not name anything specific: it refers to any nebulous but presumably devastating threat that becomes an obsession for an individual or even an entire nation, regardless of its materiality. Much of its terror comes by way of implication. Examples include the communists of the 1950s, the JFK conspiracy and Weapons of Mass Destruction.
A word in the "slanguage" used in the famous film industry magazine Variety. Other examples include "cumes" (for "cumulative totals"), "thesp" (for "thespian" or "actor"), prexy (for "president") and "perf" (for "performance").
"Though no hit with the crix, pic's boffo homevid cumes recall Fight Club, another Pitt starrer for the Frog. New prexy Weiss is also excited about new Wilson brothers laffer..."