Term originated on Live Journal (adapted to Tumblr) for Robert Downey Jr (RDJ) fans. In a video, RDJ introduced himself as Rubber Ducky Jr, to which LJ user UltraDowney (Jakob-Dylan on Tumblr) developed the nickname for the RDJ fandom.
A term invented by Brown University students (though its usage has spread to most other colleges) refering to the penis of a man who remains perpetually unerect, even when being sexually stimulated by a woman. It is often the result of intense sexual inactivity over long periods of time. Often, men with duckles can be sexually excited by other men, and tend to become homosexuals later in life.