How is “be in on the ground floor” used?

In How the West Was Won (1962), one line is: “"Cleve has hopes of getting in on the ground floor.”

Which movie is “be in on the ground floor” from?

The clip at the top of this page is from How the West Was Won (1962).

to become involved in a plan, project, etc. at the beginning

Heard in 3 movies

Oxford100% of films
to become involved in a plan, project, etc. at the beginning
And I wanna get you in on the ground floor with me.
An opportunity for a guy like me to get in on the ground floor.

Definitions from the Oxford Advanced Learner's Dictionary, 9th edition

"Cleve has hopes of getting in on the ground floor.

Said in 8 films · #1