The natural effect of the slow growth and spread of a clerical worker's physical frame, across the span of years of combining vending machine snack food with an exercise regimen that consists of little more than typing, until said physical frame is confined by the spatial limitations of the worker's desk chair. Commonly applies to programmers, secretaries, and middle management. In extreme cases, can be accompanied by a non-insignificant amount of physical exertion to free the compressed rear end from the chair.
She was really slender and athletic, before she started doing data entry 60 hours a week. But after all those late nights and candy bars, she's got a real case of Chair Ass.