The term 'become ground down or deteriorate' can be summarized by two crossword solutions: 'ERODE' (5 letters) and 'WEAR' (4 letters). ERODE means to gradually destroy or diminish something, usually through the action of natural elements such as wind or water. WEAR, on the other hand, refers to the gradual destruction of an object due to friction or use. Both terms refer to the process of deterioration over time.