How is “buck up” used?

In Sons of the Desert (1933), one line is: “Now, buck up. Go in and tell her that you're going to the convention.”

Which movie is “buck up” from?

The clip at the top of this page is from Sons of the Desert (1933).

to become more cheerful

old-fashioned · Heard in 6 movies

Oxford60% of films
to become more cheerful
Well, Edward, you'll just need to buck up.
Come on. Buck up.

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

Now, buck up. Go in and tell her that you're going to the convention.

Said in 8 films · #1