A person you hook-up with at the end of the night out of desperation.
Girl 2: Well, take that guy over there home with you; he's not that bad-looking and you can dump him in the morning
Girl 1: Yeah, I guess he's my Rice-a-Roni for tonight
i. implies the target is gay,
ii. presumes the target would be offended by that implication,
iii. refers indirectly to the city of San Francisco as home to both the food product "Rice-a-Roni" (according to their well-known 1970s ad campaigns in the USA) and a large, well-established homosexual community.
(This definition incorporates the example given by another contributor, to preserve the neutral observation of actual urban usage, in the event that the current votes to remove that definition are successful.)