Defecating on your partner's head while receiving a hummer. This move is achieved facing away from your partner while on top. Facing the other way results in a brown bow-tie.
Jethro slipped me a laxative that kicked in while Sherry and I were 69ing and she was giving me a hummer and I couldn't help giving her a brown sombrero!
a fecal hat resulting from sticking one's head up another's ass
I gave Janie a brown sombrero yesterday, and while it hurt my bum, she seemed to enjoy it.