Formerly stood for Republican In Name Only. Eventually, it fell into widespread overuse, effectively diluting its context.
Currently, it stands for any Republican who is not totally bat-shit crazy on 100 percent of the issues, 100 percent of the time.
Person A: "Senator X is nothing but a RINO!"
Person B: "Oh, he's not a RINO, he just doesn't think schizophrenics should be allowed to carry assault rifles into schools!"
Person A: "That's not right! The Second Amendment says we can carry anywhere!"
Person B: "Sure it does - if you're bat-shit crazy!"