Rent boy, or rentboy, is a term that is used to describe a particular type of male prostitute. It refers primarily to young men (although its use extends to male prostitutes of all ages) who have sex with men. The term originated in Great Britain, Ireland, and New Zealand and originally was applied to a male prostitute who was both young and working-class. Middle-class and upper-class male prostitutes more commonly would have been called escorts, and older male prostitutes would have been referred to as hustlers. The rent boy was almost exclusively a street worker who charged low rates for his service. Although the term retains its original meaning, it has gained a certain cachet within the sex-worker community and sometimes is appropriated by higher-end male prostitutes who wish to provide a rough trade fantasy to their clients.

The most famous use of the word was by Thomas Swinscow, who admitted in 1895 to having worked as a rent boy in a London male brothel. The subsequent investigation, called the Cleveland Street Scandal, implicated high-ranking officials. Rumors circulated for decades that the prince of Wales was involved in the scandal, and records released in 1975 confirmed that, although they did not conclude that he was a patron of the brothel. The public outcry over the scandal was partly responsible for the prosecution of Oscar Wilde that year on charges of gross indecency. Victorian morality could not accommodate the existence of rentboys, and rejection of the practice led to a general increase in homophobia.

https://www.encyclopedia.com/social-sciences/encyclopedias-almanacs-transcripts-and-maps/rent-boy

Pretty sure it’s a male prostitute that turns tricks to pay rent, hence the name.