To reply to a message with an image or other reference to the Star Wars character Yoda, as a way of mockingly pointing out that the message is redundant and the content has been seen before. This is usually only relevant to a public forum, in response to people who have reposted content which has already been posted, in order to mock them for not noticing that the content had previously been posted.
Origins: In fall of 2005, Shay Pierce observed two people send out the same video of Yoda rapping twice within two days to the "Not-work-related" e-mail list at his place of employment, Midway Studios Austin. Several days later, there was a separate occurrence of the same thing: one person sent out a news article which had been sent out days before. Shay responded sarcastically, saying "That's awesome... hey have you seen this video of Yoda rapping?" This began a running gag that continues to this day: anyone sending a repetitive e-mail risks being publicly yodaed. (Today the yodaing is usually done simply by pasting some image of Yoda, preferably in a "rapping" or "dancing" context.)
"Dude, that video of the Weird Al music video has been sent out three times now. I'm going to yoda this guy so hard."
"Why was I yodaed?" "Because that video of the spiders on crack was posted twice already, moron."