An explanation, sometimes a bit rueful, of the reason a person is embarking on a particular enterprise, a trip, or some other, perhaps surprising undertaking. It often refers to the fact that the person feels the approach of old age and attendant infirmities, and wants to accomplish something while it is still possible. For example, “When I asked Joe just why he was booking a trekking trip to Nepal, he said, ‘I’m turning 65 next year. If not now, when?’” But the expression also may simply refer to the fact that an excellent opportunity has presented itself, one to take advantage of, as in “That jeweler’s going out of business and I’m buying myself a diamond ring—if not now, when?”