If the shoe fits, wear it is a condensed form of the expression "if the shoe fits, wear it." It's an idiom used to persuade someone to acknowledge the legitimacy of a critique or the accuracy of a portrayal of oneself. The phrase comes from a medieval proverb that compared the fit of a shoe to the fit of a person's character. Shoes were then as now a valuable commodity and the fit between a person and their shoes could be important because of this. If you were trying to convince someone to accept you as a critic of your own character, for example, you might say that the fit was not good because they didn't agree with what you had to say. The proverb became associated with other ideas related to persuasion and debate. It can also be put in terms of health to suggest that if something is done to improve one's physical fitness, one should wear it even if it isn't particularly comfortable or fits well. Finally, the phrase can also be used as a metaphor for situations where one has no choice but to accept something: if the horse you're riding doesn't fit, you have no option but to carry it. These are just some of the ways in which the phrase "if the shoe fits, wear it" has been used over time.