grindstone 的意思是磨刀石，古代会放在木头上，把整块石头像轮子一样转动来磨利刀具、武器等等。那可能磨刀时鼻子要靠近磨刀石，又要磨很久，keep one’s nose to the grindstone 就引申为埋头苦干、长时间苦干、努力做某事情、锲而不舍。

【例句】
She kept her nose to the grindstone for half a year and got the result she wanted on the IELTS test.
她苦读了半年书，终于取得了满意的雅思成绩。

I've got to keep my nose to the grindstone if I'm going to get this promotion.
如果我想升职，我就得埋头苦干。