godan verb (-tsu), intransitive verb
become fluffy; become fuzzy; become nappy; become frayed
Describes fabric, yarn, or similar material developing a fuzzy or napped surface, often from wear or washing. Also used for hair or fur becoming rough or standing up.
このセーターは何度も洗ったので毛羽立ってきた。
This sweater has become fuzzy after being washed many times.
Towels can become nappy if you put them in the dryer.