Hamstring can mean two different things:

1. The group of three muscles and their tendons running down the back of your thigh, from your hip to just below your knee. These muscles help you bend your knee and move your leg backward. Athletes often injure their hamstrings during sprints or sudden movements: a hamstring injury can sideline a runner for weeks. You might hear a soccer player say she “pulled her hamstring,” meaning she strained or tore one of these muscles, which is quite painful.
2. To severely limit someone's ability to act or succeed. When budget cuts hamstring a school's art program, administrators can't buy supplies or hire teachers, crippling what the program can accomplish. A new rule might hamstring your ability to complete a project the way you planned.

When someone or something is hamstrung, they're stuck and unable to move forward effectively, like a person whose hamstring muscles aren't working properly.