Definition of big one in English:

big one

noun

informal
  • 1British US Originally and chiefly US. A large sum of money, especially one thousand (occasionally one million) dollars or (British) pounds; a note of large denomination. Also (usually plural): one dollar or (British) one pound.

  • 2US Originally US. With the. Something regarded as important and decisive, or as the most significant, substantial, or influential of its kind; especially a factor, event, opportunity, etc., of great potential or consequence.

  • 3A war, originally specifically the Second World War (1939–45), now frequently a prospective nuclear war. Hence: a major military incursion or assault.

  • 4A major disaster, specifically a (prospective) large-scale earthquake.

Origin

Mid 19th century; earliest use found in Tom Taylor (1817–1880), playwright and comic writer. From big + one.

Pronunciation

big one

/ˈbɪɡ wʌn/