The phrase developed out of the 2004 U.S. presidential election, when a group called "Swift Boat Veterans for Truth" attempted to suggest that Democratic candidate John Kerry lied in order to earn two Purple Hearts and a Bronze Star during the Vietnam War. The group, as it turns out, was funded primarily by people who also frequently donated millions of dollars to the Republican party. Swift Boat Veterans for Truth was formed for the sole purpose of discrediting Kerry's Vietnam War service and has not been heard from since the end of the election.
refers to the swiftboat veterans for truth. when john kerry attempted to capitalize on his brief service in vietnam, a group comprised of all but a handful of veterans who served in the same combat area of operations and mos came forward to underline what a bullshitter kerry was. among other things, they demonstrated that kerry didn't earn his combat medals, lied about christmas in cambodia, and seemingly only enlisted so he could turn directly around and participate in an effort to brand all veterans as war criminals.
the term has since been turned into a generic pejorative for any revelation about a public progressive which progressives can't dismiss on a factual basis. the idea is to name the "crime" of telling the truth about progressives on the same axiomatic level as "theft" or "rape". progressives are implicit totalitarians who don't like having their authority challenged.