Definition of make it rain in US English:

make it rain

phrase

US
informal
  • 1Throw paper money in the air so that it showers down on someone, especially a stripper who is performing.

    ‘he's making it rain at the local strip joint’
    1. 1.1 Spend money in a lavish or extravagant manner.
      ‘even people who carefully budget their money when working for low salaries have a tendency to start making it rain as soon as their paychecks get bigger’