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

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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.

Definiciones del Oxford Advanced Learner's Dictionary, 9.ª edición

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

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