Definition of big one:
(n) :
(colloquial) Something important; (with 'the') the most important one, (especially sports) the big game, the big play.
(n) :
(US, colloquial) One hundred or one thousand dollars.
(n) :
(US, colloquial) A dollar.
(n) :
(slang, originally US, euphemistic) Death.
(n) :
(seismology, colloquial, usually with the) A hypothetical massive earthquake somewhere along the Pacific coast of North America, but especially one projected to cause catastrophic damage to a major city such as Vancouver, Seattle, San Francisco, or Los Angeles.
(n) :
(colloquial, by extension) Any other hypothetical event with the potential to be far more devastating than previous events of the same type.