big one

noun4

1. •
   (informal) Something important; (with 'the') the most important one, (especially sports) the big game, the big play.

   • Wood agreed. ‘This is it.’ ‘The big one,’ said Fred Weasley. ‘The one we’ve all been waiting for,’ said George. ‘We know Oliver’s speech by heart,’ Fred told Harry. — 1997
2. •
   (US, informal) One hundred or one thousand dollars.

   • “Little Caesar stopped by. You guessed it. Edward G. Robinson himself, and paid four big ones for the seated figure. […]” — 1988
3. •
   (US, informal) A dollar.

   • The visitors won't know the difference because […] after they’ve dropped five hundred big ones at the factory outlet stores, an extra dollar will seem like the bargain of the century. — 2007
4. •
   (slang, euphemistic) Death.

noun2

1. •
   (informal) 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.
2. •
   (informal) Any other hypothetical event with the potential to be far more devastating than previous events of the same type.

   • Oh, it's the worst one, this is the Big One [heart attack]! I'm dyin! You hear that, Elizabeth, I'm comin to join ya, honey [in Heaven]! — 1972