A homosexual term for when one guy butt fucks another guy. The "mud" is a crude term for shit in the bottom's anus. The "helmet" is to describe the shape of the head of the penis. When the top (helmet) rams his dick into the anus of the bottom, he gets shit (mud) on it. Thus, "mud on the helmet".
"I fucked my boyfriend so deep I got mud on the helmet."
by rjc July 08, 2006