If you don't practice your English often enough,
如果你不经常练习使用英语，
one day it will become rusty.
有一天你的英语会生锈。
Likewise, if you idle around doing nothing,
同样地，如果你游手好闲无所事事，
your mind will become dull.
那么你的心智就会变得很迟钝。
There's a proverb that goes:"Idleness is the rust of the mind. "
有道是：“游手好闲使心智生锈。”