Like Bi, but not. Willing to engage in sexual activity with anyone or anything.
Despite it being a fashionably nonsensical new version of “bisexual”, the modern, post-Freudian use of the term can be traced back to the 1980 Hudson Review, which describes “Mr. Maupin’s people, mostly San Francisco homosexuals (of both sexes) or bisexuals or pansexuals”. The pansexual “flag” was added to the LGBT “rainbow” in 2010.