This, actually means that you want to start a fight. A looong time ago people used to "bite" their thumb at someone to show that person you don't actually care about them.
I bite my thumb at thou!. I completely disagree with you all the way.
When armies took prisoners in southern Europe in the middle ages, they would sometimes be offered the chance to earn their freedom by eating a fig from a mules anus. As the captors taunted their captive, offering him this degrading method of escape, they would bite their thumbs.