The "Boot Knife" is a term which here refers to a boot with a knife sticking out of it. It is currently believed that the term originated during an extralives gaming marathon for charity, when it was pointed out that the symbol on the doors of the Pirate's Fortress in Majora's Mask looked like a boot and a knife. The meme has since grown to a viral level, rapidly becoming one of the internet's most popular memes.
Exactly how it sounds. A boot - or possibly a shoe - with a knife protruding from it. The knife may point in any directions, but straight up is preferable. It's origin spawns from the doors in the pirate temple of Majora's Mask. They portray an arm holding a knife, however it has been perceived as a bootknife.