Originating at the moment of excitement and adrenaline rush when a person was first able to touch a basketball rim, get rim simply means to get big, or get pumped up, as if before a big game or before going out on a date with a hot girl. If used in context with sports it does not have to be for basketball only, it can be used for any sport
-"dude the state championship match is tonight man. I am pumped!"
-"Yeah dude you got this in the bag. Get rim!"