No needs to be sorry, not too annoying, Yes your right, it could mean many things, as I see you checked out wikipedia too. However as per all the spam i get in my inbox everday, Rx seems to be most popular for prescription medication. But you could use if for anything u really want, hell it can even mean EZ Rachel Xavier if you really wanted it to. :)
Sorry to be annoying, but I can think of another meaning: RX can also stand for Recieve (as opposed to TX which stands for Transmit) in the world of radio and data transmission.