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“It’s A Trap!” is a catchphrase that is often used as a reaction to photos of transsexuals and crossdressers (often referred to as “traps”), or people who appear sexually ambiguous. It usually means that the person in question has male reproductive organs, regardless of their appearance. Though less frequently, the phrase can be also used as a warning of any potential danger.
Admiral Ackbar: “Take evasive action. Green Group, stay close to holding sector MD-7.”
Crewman: “Admiral, we have enemy ships in sector 47!”
Ackbar: “It’s a trap!”
The phrase comes from a quote in the film Return of the Jedi said by the Mon Calamari rebel leader, Admiral Ackbar. When the Alliance forces attempt to blow up the DeathStar, they are met with an ambush that leads the Admiral to exclaim, “It’s a trap!”.
An image macro was created by Something Awful admin “OMGWTFBBQ” featuring a screenshot of Admiral Ackbar with the text overlaid “IT’S A TRAP”. It spread to Fark sometime in 2002 (the earliest Google search results are from August), and was commonly used in photoshop threads as a running joke. It didn’t become associated with transsexuals and crossdressers until later, when it reached 4chan.
It’s a Tarp!
“It’s a tarp!” is an intentional misspelling that spawned one of the more frequently posted derivatives seen above. An Urban Dictionary entry was created on May 20th, 2009:
A phrase based on a misspelling of “It’s a trap” a phrase and meme said by Ackbar in Star Wars Episode VI. It is popularly used sarcastically on message boards and chan sites.
/b/tard 1: It’s a trap!
/b/tard 2: No! It’s a tarp! 
According to Google Insights, search queries for “it’s a tarp” picked up in April of 2008. Several months later the “Troubled Asset Relief Program” also known as TARP, inspired several derivatives.
Family Guy Episode
The finale of the 9th season of Family Guy was an hour long episode titled “It’s a Trap!”,and was a spoof of Star Wars: Return of the Jedi that was released on DVD in 2010, prior to it’s expected May 2011 air date.