expression, godan verb (-ru)
become less capable; lose one's touch; get rusty
Used when a skill or ability declines due to lack of practice. Often implies a former high level of proficiency.
しばらくピアノを弾いていなかったので、腕が鈍った。
I haven't played the piano for a while, so my skills have gotten rusty.
毎日練習しないと腕が
If you don't practice every day, you'll lose your touch.