A burnt sienna crayon is where you have anal sex with a girl and while you're doing it, you fuck her so hard that she bleeds in her ass, and then you piss inside her ass and keep fucking. This will cause the blood and piss to mix together forming an orange like colour (burnt sienna) and the girl will take a shit onto the floor. This piece of shit is called a burnt sienna crayon which you use to write your name on her mirror before leaving.

A reference to the burnt sienna crayon is made in the show family guy in the episode "the Cleveland Loretta Quagmire":

Cleveland: "Hey baby, how'd you like to go black and then have to make a difficult decision regarding whether or not to go back?"
Chick in bar: "Sorry, I went burnt sienna and never went back."
Crayola Crayon: "Ready baby? Who's this clown."
Damn i left Julia's place this morning and forgot to give her a burnt sienna crayon. Hope she answers my calls.

Woman: Hey mom, please come over a little later today as I have to clean up the burnt sienna crayon these guys did to me last night.

Mom: Your father used to do that to me when we were dating, that's how you were born darling!
by seppoporn November 3, 2009
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