Definition of big money:
(n) :
(informal, idiomatic) A large amount of money, especially a significant source of revenue or income.
(adj) :
(informal, idiomatic) Involving or transacting a large amount of money.
(n) :
(informal, idiomatic, politics) Large corporations, the people who run them, or corporate interest generally, seen as exerting political influence and prioritising profits over other political concerns.