It is an expression used in the, "if it hit you", manner. As in something you currently do not realize or experience. It literally means if something bluntly hit you in your butt/ass cheeks.
"You wouldn't know loyalty if it hit you in your cheeks, homie." - Quote or similar quote to a line said by Carl Johnson to Ryder in GTA: SA
by chickewingzbruh November 22, 2016