OK, I looked it up on Urban Dictionary.
1. Napoleon's Hat
A man vaginally penetrates one woman while orally performing upon a second woman. The two women engage in sexplay from the waist up.
"That guys loves to wear Napoleon's Hat with as many ladies as possible"
Sounds like fairly common threesome activities to me. I would speculate that it needn't be the man on the bottom, as a woman is more than capable of performing oral on another woman while getting penetrated herself.
Good times for everybody!