A humorous way of spelling "retarded." The implication is that the speaker knows that they're already anticipating being mocked for their own supposed 'retardedness', and wants to goad and provoke others into accusing them of being ignorant by changing the spelling to such an extent. Can also be used as a joke in the debate on just how offensive the word 'retarded' actually is.
Erica: Oh my fucking god, that is so retronded, I don't even know where to begin.
Brett: 'Retronded?' And you're the one going around accusing others of being retarded? Holy shit.
To make a picture, space or way of life retro! When you decide to bring back a saying from years ago, you've retroized your lingo! When you put up a 70's wallpaper, you've retroized your space, when you add a filter to make a photo look like it's from another time, you've retroized!