Which of the following terms describes the action of making or becoming overly thin?

The term that describes the action of making or becoming overly thin is "emaciate." This word specifically refers to the process of losing flesh or body mass, often to the point of being unhealthy or extremely thin. It is commonly used in medical or clinical contexts to describe significant weight loss, particularly in cases related to malnutrition or certain diseases.

In contrast, while "wane" generally implies a decrease in size or intensity, it does not specifically refer to thinning in the context of body mass. "Thinning" itself could imply a reduction in thickness or density but is not specifically focused on the aspect of becoming excessively thin. "Reduce" denotes a general decrease but lacks the specific connotation of becoming unhealthily thin, which is critical in the context of the term being sought. Therefore, "emaciate" is the precise term that captures the intended meaning of becoming overly thin.