1.应该是What do you mean by doing that?对。
2.return用法:1“返回”,相当于“go back / get back / come back.. 如:Ann will visit you when he returns to London.
(当“返回”时是不及物动词,先加to才可再加地点。另外,return已含有back的意思,后不可再跟back.)
2“归还”,相当于“give back”如:I have returned the dictionary.
(当“归还”时是及物动词,后直接加物,若再接人时方可加to. 同样不可再跟back. 如:You should return the piano to Dick on time.)
What do you mean by doing that?