How is “swear by” used?

In You've Got Mail (1998), one line is: “Salesmen swear by her. If she likes the book, it sells. Period.”

Which movie is “swear by” from?

The clip at the top of this page is from You've Got Mail (1998).

Phrasal verb

to name sb / sth to show that you are making a serious promise

Heard in 444 movies

1
swear by sb / sth
to name sb / sth to show that you are making a serious promise
I swear by almighty God that I will tell the truth.
2
swear by sb / sth
to be certain that sth is good or useful
She swears by meditation as a way of relieving stress.

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

Salesmen swear by her. If she likes the book, it sells. Period.

Said in 8 films · #1