big one
/ˈbɪɡ wʌn/informal
noun
1Originally 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.
2Originally 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.