A friend of the bosom & A bosom friend
The word below A and B are all correct to say?

A. a friend of the bosom
B. a bosom friend

(A is what I ran into from a certain American film and B is from English dictionary, which means 'close friend')

If they are all correct, are the following two sentences perfectly correct to say?

a. She seems to have become bosom friends with your children.
b. She seems to have become friends of the bosom with your children.

Origianl text:
M: Tell me, is Mrs. Van Hopper a friend of yours or just a relation?
W: No, she's my employer. I'm what is known as "a paid companion.
M: I didn't know companionship could be bought.
W: I looked at the word "companion" in the dictionary once. It said, "a friend of the bosom."

Question

A friend of the bosom & A bosom friend
The word below A and B are all correct to say?

A. a friend of the bosom
B. a bosom friend

(A is what I ran into from a certain American film and B is from English dictionary, which means 'close friend')

If they are all correct, are the following two sentences perfectly correct to say?

a. She seems to have become bosom friends with your children.
b. She seems to have become friends of the bosom with your children.

Origianl text:
M: Tell me, is Mrs. Van Hopper a friend of yours or just a relation?
W: No, she's my employer. I'm what is known as "a paid companion.
M: I didn't know companionship could be bought.
W: I looked at the word "companion" in the dictionary once. It said, "a friend of the bosom."

Answer

This film conversation is more or less the same dialogue as in the original Daphne du Maurier novel 'Rebecca', including the phrase 'a friend of the bosom'.

It's a very odd expression, and it's definitely not an accepted term in standard English. You can say 'bosom friend' or 'bosom buddy', but not 'friend of the bosom'. It sounds like a joke, or the type of thing a French or other foreign character in a book might say - getting an idiom slightly wrong to comic effect.  I would forget about it if I were you.

Answer

SuKi's answer is excellent. "Friend of the bosom" was intended as a joke.

In the US, we use "bosom buddy" not "bosom friend".

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