Why is the supreme court called the court of last resort?

The term "court of last resort" generally refers to the Supreme Court (in the USA) or the highest court in the land (in other countries). This nickname applies because you cannot appeal any higher than the Supreme Court. It's literally your last resort - your last option for having the outcome of your case changed. If the Supreme Court turns down your appeal or decides against you, there is no higher court to which you can appeal.