From Longman Dictionary of Contemporary Englishif you must (do something)if you must (do something)LET/ALLOW spoken used to tell someone that they are allowed to do something, but that you do not approve of it or agree with it All right, come along, if you must. If you must smoke, please go outside. → must
Examples from the Corpusif you must (do something)• But if you must..• Very well, she'd tell them, leave if you must, but I want no abuse, is that clear?• What good was freedom if he must give up his home to win it?• These people here have so little conception of our world that sometimes I feel myself as if I must have dreamed it.• It's better not to use a flannel, but if you must, keep a separate one. 6.• "Who was that girl?" "Well, if you must know, her name is Mabel."• But if you must know, Mrs. Jewkes was present.• But I was extremely jealous of him, if you must know.• Look, if you must know!• If you must travel at night in dangerous areas, turn on the interior light in your car.