An act committed by a dominant gay male, or a top, whereby he sneaks his hot dog in-between your buns by planking on your back; fully erect.
The flat top is considered to be one of the most daring and skillful planking maneuvers, as well as one of the most demeaning gay sex acts.
Guy 1: "I passed out drunk on the yard last night while taking a piss, and Chris took pictures planking on my back naked. On an unrelated note - Jagermeister really makes your asshole hurt in the morning."
Guy 2: "Dude... I saw the pics on Facebook.... you got Flat Topped. The national sexual assault hotline is 1-800-656-HOPE."