A Crank is a slang term for someone who has been infected by the Flare virus.
Series
At the end of the story, as the Gladers are rushing to the bus, Thomas is confronted by a female Crank, who asks crazily if he is going to save the world from the Flare. Thomas is terrified at the sight of her. Thomas is then ushered onto the bus away from the woman, and as the bus leaves it runs her over, causing the driver no apparent concern.
As the Gladers are crossing the Scorch they come upon a seemingly abandoned city. However, as they take shelter in one of the buildings they are confronted by a bunch of Cranks, who reveal that the city is full of Cranks at various stages of insanity. It is revealed that the city is a dumping ground for some Cranks, who are sent there so as not to spread the Flare any further.
In the movie, the group encounter many past-the-Gone Cranks in the city, who have strange vine-like growths protruding from their bodies. Teresa also tells Thomas how her mother caught the Flare and went insane.When they reach Denver, Thomas, Newt, Minho, Brenda and Jorge find that the city is walled and secured from the Cranks. Any citizens who catch the Flare are immediately sent to the Crank Palace, where Newt is also sent. The Palace is filled with Cranks, many dangerous and angry at the group. Later, a group of the Cranks escape the Palace and break into the city. A group of them block a highway. Thomas encounters Newt there once again, and fulfills Newt's final request written on the note he left for Thomas by shooting him in the head before he goes completely insane.
Alec tells Mark that the way the people in the street are behaving reminds him of a Crankhead party. Since no other mention of the origins of the name Crank are mentioned throughout the series, the most likely origin is "Crankheads", who were users of a certain drug in the twenties. The drug eventually made people completely crazy, and they never recovered. The Toad mentions that he has things in his brain, which was one of the first symptoms of him being a Crank.