As a Noun
1. The feeling of being unhappy, usually because something has happened that you do not like. English = sadness.
2. The feeling of being unhappy because you have lost something very important to you, particularly a loved one who has died. English = grief.
As a Verb or Adjective
1. To be unhappy, usually because something has happened that you do not like. English = (be) sad.
2. To make someone feel unhappy, usually by doing something that they do not like. English = sadden.
1. As with most signs for emotions and feelings, the expression on your face usually shows the meaning too. But you don't always have to add an expression.
1. It would seem the more common form of SAD moves in the opposite direction (click on the NEXT button above and go to the next sign in the dictionary to view the video).