Mopey, in slang, is defined as 'emo, ' which is a term used to describe someone who is emotionally sensitive and often expresses sadness or depression. It is derived from the word 'EMOTIONAL' and is often used to describe people who are introverted and have a pessimistic outlook on life. The term is often used in a derogatory manner, but can also be used to express sympathy or understanding for someone who is feeling down.