Is when a group of people, usually male, are chillaxing and one of them farts. The d-bag of the group will then proceed to yell "OMG! Someone blew a fattie!"
This is always followed by the rest of the group laughing and pointing and attempting to pass the blame.
People in the general vicinity will also point and laugh; not in an attempt to pass blame, but to make fun of them.
Chris: OH MY GOD! SOMEONE BLEW A FATTIE! (He was the actual one who took partin the action known as "blow a fattie")
(People around him begin to laugh and point and pass blame)
Intelligent Person 1: Whoa, what losers.
Intelligent Person 2: Fo sho.