The line comes from The Legend of Old Gregg, a 2005 episode of the British comedy series The Mighty Boosh. Old Gregg is a flamboyant, lonely merman-like creature who lives beneath Black Lake and confronts fisherman Howard Moon after Howard hooks him. Played by Noel Fielding, the character became one of the show's most quoted creations, and this territorial question spread through forums, videos and soundboards as a shorthand reaction. Its odd mix of threat and vulnerability keeps it popular long after the series ended.