Mum's the word (I'll mum the word)

[Mnemon]

A. You won't tell anyone what I told you, will you?
B. No, I promise. Mum's the word.

English Idioms in Use Advanced book

Do you think the expression has been used correctly in the above quote? Would it be wrong if the speaker B said I'll mum the word?

 

[5jj]

It's used correctly. Your version is not possible.

Definition of MUM'S THE WORD

—used to say that some information is being kept secret or should be kept secret… See the full definition

www.merriam-webster.com

 

[Mnemon]

Thanks. Is it a fixed expression? I mean, is it always used in the form mum's the word?

 

[5jj]

Yes.