Green’s Dictionary of Slang

1. (US black) a man who is conned into paying in general.

1958

Hughes & Bontemps Book of Negro Folklore 487: sponsor [...] How’d you let that sponsor go before he paid the bill.

2. (US gay) an older homosexual man who supports his young lover.

1972

B. Rodgers Queens’ Vernacular 191: older man who supports a younger lover [...] sponsor.

3. (US prison, also financier) a man who sends money and gifts in to a female inmate.

1992

Bentley & Corbett Prison Sl. 40: Sponsors also Financiers Men who send money and other items to women in prison.