Holden means that his brother is working as a film writer and that this is a form of prostituting his talent, that is, doing something purely for money and not for genuine love of art or creative self-expression. It is common for people to be called prostitutes, and even to consider themselves prostitutes, if they are working at something they despise and doing it solely for money. Holden might have said that D.B. was being a hack. The term "hack writer" has become a more common tag than "prostitute," but it means about the same thing. Holden may have picked up the notion of his brothers' being a prostitute from his brother, who despised what he was doing but liked the big money to be made in Hollywood and was being morally undermined by the work he was doing and the cynical people he had to associate with. Many talented novelists took work in Hollywood and despised the industry and themselves for turning out cheap, insincere dramas. These included F. Scott Fitzgerald, Raymond Chandler, Dorothy Parker, and William Faulkner. What is especially demeaning about being a hack writer in Hollywood is that the writer has to take orders from inferior men who control the purse strings.