Hi LiZebin "Consider it done" is used to say that one will happily do something that he or she has been asked to do: Ex A) Can you go to the shop for me. I don't feel that well B)Yes of course love, Consider it done A)Can email Sarah for me Jude?I can't right now. B)Consider it done boss."

Hello LiZebin I love this expression. When you say that, you actually tell the other person that he or she can be absolutely sure that you will do as promised. It's as if it has already been done. So "you may consider it done."