The Unforgiving Servant is a man who was mentioned by Jesus Christ in Matthew 18. This man had refused to extend mercy to another after mercy had been extended to him, angering his master in the process and getting him into lots of trouble.
According to Matthew, a master decided to do some bookkeeping and found this servant owed him a lot of money. He decreed the man be sold, along with his family and his possessions to settle the debt. The man begged the master to give him more time to pay. Then, moved by the servant's pleas, the master let him go and forgave him his debt.
A short time later, however, the forgiven man came across another man who owed him a much smaller amount. Instead of showing mercy when this man asked for more time, the servant had the man thrown in prison.
When the master found out, he was not very happy, and had the servant hauled in. The master released the fellow servant and then called the unforgiving man a Wicked Servant, and blasted him for his refusal to show the same mercy he was shown. The master then had the servant handed over to torturers until he coughed up his debt. According to Jesus, this is like how God will treat anyone who doesn't forgive others.