"Scrooge" is a term derived from the Charles Dickens' story, A Christmas Carol, which is about a character named Ebenezer Scrooge, a man who is rich, unwilling to share his money with the poor, grumpy, and unappreciative of any aspect of the Christmas holiday. Eventually, the story became very popular worldwide, and people started using the term as a kind of insult to describe many different behaviors similar to the character's in the story. The term has generally become popular enough so that some people may use it without knowing very much about the original story at all.
In the story, Ebenezer Scrooge is an elderly shop owner with lots of money. He is unwilling to help his workers in any way, not even those with very difficult lives, and he refuses to make special exceptions for the Christmas season. While he sleeps, supernatural forces congregate to teach him a lesson. He has visions of several ghosts, representing different points in time, and they show him all sorts of important moral lessons that eventually help him to become a better person. When people use the character's name as an insult, they generally apply it to someone who has demonstrated one or more behaviors that line up with those seen in the first part of the story.
Many people call someone a scrooge because the individual has shown a tendency for greed. This is especially true when the greedy behavior specifically affects those who are less fortunate. For example, if someone raises prices on a crucial item at a time when people desperately need it, that person might be described in this way. The term could also be used if someone were to run a business and underpay his employees even though they worked very hard and helped him make huge profits.
Another common behavior associated with the term is a lack of compassion for those living in poverty. This could include those with lots of money who refuse to give to charity. Another example would be someone who refuses to give a loan to a friend in need.
One more key aspect of scrooge-like behavior is a lack of appreciation for the more joyful things in life. People use the term to describe people who are grumpy and generally unhappy. For example, a person who refuses to return another person's smile or a someone who yells at his subordinates rudely could be called by this name. It is also used to describe people who hate any kind of holiday experience, especially those who don't enjoy the Christmas holiday specifically.