Hand over fist

"Making money hand over fist" means to make a lot of money quickly.

 This term has a nautical history. It relates to the practice of climbing a rope hand over hand. This soon became known as hand over fist, with the fist being the hand gripping the rope. The term hand over fist soon evolved from making progress up a rope to making progress generally. Today, it relates only to financial gain.

Fonte: Grammar Monster