The word “function” is applied to people in a variety of ways.
Job descriptions detail basic functions of the position, and mental health professionals declare people high- or low-functioning.
If my goal is to be a “functioning” student, employees, husband or son, my value as a human being is tied up with what I’m doing. That “doing” is generally for the benefit of some social institution or for the sake of someone else’s bottom line.
You can imagine what it might feel like if, through disability or happenstance, one is no longer “functional” in one area or another.