“Lability” is derived from the Middle French term “labilité,” based on the Latin “lābilis,” meaning “apt to slip.” In its original usage, the word “lability” didn’t refer to the potential for a person or thing to change. Instead, in the 16th century, “lability” measured a person’s likelihood of slipping into error, sin, irrationality, or bad behavior. By the 1700s, the term had come to mean something closer to “instability.” A neutral definition of “lability” (related to changeability) won out, and today, “lability” is most commonly used in psychology as a term for emotional fluctuation and responsiveness.