In a Shakespearean context "Willpower" or Will-power" or "WillPower" is a play on words often used to refer to the power of William Shakespeare. In 2019, for instance, the Pennsylvania Shakespeare Festival included "The WillPower Tour" (website attached). Since the pun is a fairly obvious one, it has also been used in the titles of various books and articles, alongside such variants as "Where there's a Will, theres' a way" and "The Will to Win."

In Shakespeare's time, Will was the most common contraction of William, and Shakespeare puns on it himself in the sonnets. The term "Willpower" has a wide range of applications, and could refer to the power of language, the power of drama or performance, or the powerful themes addressed in Shakespeare's plays and poems.

Various T-shirts are in fact available with "Willpower" or some variant, usually written below a picture of Shakespeare's head. Wearing such a T-shirt might mark you out as someone interested in Shakespeare, literature, drama, or the power of language. However, it might mean nothing, as such t-shirts are often given as novelty gifts. Even in this case, there is perhaps a suggestion that the wearer at least knows someone who is interested in Shakespeare, and is close enough to give them a t-shirt.