Why do they call it a quarterback?

Answer 1

From Wikipedia: "The term quarterback has its origin in Scottish Rugby, wherein backfield players, according to their customary distance behind the forwards, were designated 'quarter back' (i.e. ¼ of the way back), 'halfback', and 'fullback'."

Answer 2

In the earliest days of American Football, the backfield consisted of four players behind the linemen. The one farthest back from the line was called the fullback (fully back from the line). Two players were positioned halfway between the linemen and the fullback and were called halfbacks. The man closest to the line was positioned one quarter of the way between the linemen and the fullback and hence became the quarterback