When you shoot a gun, fire comes out the end of the barrel(the muzzle). This is the muzzle flash. Usualy it's how you can find someone shooting at you.
In world war one, most soldiers couldn't see the enemy, they just shot at the enemy's muzzle flash.
When a male shows just the end or (Muzzle) of his penis to a crowd. Muzzle Flash
Dude that guy just muzzle flashed the party. What a fag.