In Shakespearean times they used to bite their thumb at people (the whole first scene of Romeo and Juliet involves a fight over this obscene gesture). I'm not sure if thumbing your nose at someone evolved from that same gesture, but it's a possibility. At the very least, biting your thumb at someone is a disrespectful gesture that is at least 500 years old.

Yup. Also known as "the fig." Variants of biting the thumb include thumbing the nose, brushing the hand outward at the bottom of the chin, etc. Scalia used that one recently.