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.

The safeguarding of Earth and other worlds from biological cross-contamination (i.e. billionaires with too much time on their hands).

Also known as “planetary protection.” Planetary protection / quarantine “reflects both the unknown nature of the space environment and the desire of the scientific community to preserve the pristine nature of celestial bodies until they can be studied in detail.”

There are two types of interplanetary contamination. Forward contamination is the transfer of viable organisms from Earth to another celestial body. Back contamination is the transfer of extraterrestrial organisms, if such exist, back to the Earth's biosphere.

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.

A boot with a knife, or, a knife with a boot.