The word "毛病" is formed by combining the characters for "fur" (毛) and "illness" (病), originating from classical Chinese veterinary terminology where it specifically described diseases that manifested in an animal's coat, such as mange or other superficial ailments. Over time, this literal meaning expanded metaphorically to denote any minor flaw, defect, or quirky problem, whether in objects, systems, or human behavior, thus evolving from a concrete physical symptom to a broad expression for shortcomings.