In the film "Taxi Driver", what was the nickname Jodie Foster's character had for Harvey Keitel's character?

In the 1976 film "Taxi Driver", the nickname Jodie Foster's character (Iris) had for Harvey Keitel's character was Sport. In the movie, the key character Travis Bickle (a 26-year-old honorably discharged U.S. Marine) is disgusted and depressed each time he sees the sleaze, dysfunction, and harsh living taking place in New York City.

He feels further frustrated when the adolescent prostitute and runaway, Iris, enters his cab. She is attempting to escape her pimp, Matthew "Sport" Higgins. Sport drags Iris from the cab and throws Travis a crumpled $20 dollar bill. After that incident each time he sees a $20 dollar bill, it reminds him of Iris and all the corruption that surrounds him each and everyday in all aspects of his environment.