[dào dǐ]
（到盡頭） to the end; to the finish
fight to the finish
打到底
hold on to [till] end
堅持到底
The experiment finally proved to be a success.
試驗到底成功了。
What on earth did you say?
你到底說了些什么?
Don't get discouraged, you are new to the work after all.
別灰心， 這工作對你說來到底還是生疏的。