What does "you @55 is grass" mean? Seriously, I don't get it. Like, "grass," why "grass"? How is calling someone's butt "grass" insulting?

It's not a comparison.  It's a death threat. Originated from the fact that a murdered person's body decomposes and then provides fertilizer for grass, hence the "a$$" becoming grass

  The full line is "Your A** is Grass and I'm the lawnmower!" meaning that the person doing the speaking is going to take them on and cut down or  destroy them somehow the way a lawnmower cuts down grass.