Usually one which likes to suck on someone elses penis while it is floppy. The term flopslopper usually only points towards homosexuals, but can sometimes be those which just have a craving for penis.
man 1: Did you hear about your brother last night and his boyfriend?
man 2: Oh yeah, they slopped eachothers flops.
man 1: Geez, did you see that guy back there?
man 2: Yeah, hes a total Flopslopper.