Unfriendly fire is when you fart on a friend as a joke. Unknown to you, this certain friend also happens to have a nasty fart brewing, so they release it upon you as immediate revenge
"Last night when I was hanging out with my friends, I decided to rip a fart in Franks face. Unknown to me, I was about to be hit with some unfriendly fire."
by Punchy McAssface Jr. May 11, 2010