Imagine you've just eaten a flaky croissant for breakfast you are covered in pastry crumbs. Before you go to work, you 'brush yourself off' - brush away the crumbs so that you look presentable. You leave the crumbs on the floor of the cafe, walk away, and then get on with your day. This is a literal meaning of the phrase. When someone is unhappy about something that's happened, we tell them to do this metaphorically. Wipe away the tears, straighten your tie (if you're a man), re-apply your lipstick (if you're a woman), and walk away, leaving the metaphorical crumbs behind you. Forget about the bad experience, and get on with your life.

Yes, "brush yourself off" can be said to someone who is sad or discouraged because he or she may have made a mistake or failed in someway. When we say "brush yourself off" it means to forget about your past mistakes and move on.

Hi Emma. I think of it as saying Shake it off. meaning let it go. Say it was something bothering you if you put it behind you and don't let it trouble you. Don't keep thinking about it and move forward. Maybe someone said something to you that was hurtful and made you sad, just you let it go, shake or brush it off and eventually you will forget about it completely. Probably very good advice for all of us. Hope that helps Jean