The term 'become unusable' can be described by three crossword solutions: ROT (3 letters), wore (4 letters), and DEAD (4 letters). ROT is a verb meaning to decay or decompose, usually due to the action of bacteria or fungi. Wore is a verb meaning to have used something until it is no longer usable. DEAD is an adjective meaning no longer functioning or alive. Together, these three words describe the process of something becoming unusable.