What does "hand over fist" mean? Is it an idiom?

[Omani]

English Idioms and Expressions, Intermediate level

ESL/EFL Test #19 "Idioms with Hand", question 3

This last year, my company has made money hand over fist!

(a) with a lot of struggle
(b) at a huge rate
(c) poorly
(d) distribution

English Idioms and Expressions, Intermediate level

ESL/EFL Test #19 "Idioms with Hand", answer 3

This last year, my company has made money at a huge rate!

Correct answer: (b) at a huge rate
_________________________

can you explain it.

Omani

[Mister Micawber]

.
Hand over fist = speedily; increasingly; at a tremendous rate.
.
_________________
Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mister Micawber