From Longman Dictionary of Contemporary Englishif you must knowif you must knowspoken used when you answer a question that you think someone should not have asked, because it is slightly impolite Well, if you must know, I’m thirty-six. → must
Examples from the Corpus
if you must know• April 14, if you must know.• But I was extremely jealous of him, if you must know.• I had a bad dose of influenza, if you must know.• It's my sister Rose's birthday on Sunday, if you must know.• More than enough, if you must know.• But if you must know, Mrs. Jewkes was present. 