This term originates from the parable told by Jesus in the Bible about a Samaritan (a man from a country called Samaria) who helps out a Jewish man who is ambushed by theives and left to die. This was all the more selfless of an act considering that the Jewish and Samaritan people in those times usually did not associate with each other. This story was told in response to a question concerning the scriptural mandate to "love thy neighbor as thyself". According to the story, a man asked "Who is my neighbor?" and the moral of the parable is that everyone is really your neighbor (meaning you should help anyone you see in need).
Today a Good Samaritan is usually someone who goes out on a limb to help others, even if they are complete strangers.
Whilst novices might sit on ones hand for a prolonged period, experienced operators would opt for a tourniquet.