F.B.A.T.A
Zach using F. B. A. T. A means your butt is so flappy it looks like a sad balloon
Zach: F. B. A. T. A! Your butt is like a balloon that lost its air.
Zach texts: F. B. A. T. A. Your butt is a deflated party balloon.
Zach yells: F. B. A. T. A! Your butt is a sad balloon at a happy party!