What does it mean to bite your thumb at somebody and who is Rosaline in Romeo and Juliet?

Biting your thumb was an insult equivalent to giving the finger, and I think Rosaline is the chick whose pants Romeo is trying to get into, before he meets Juliet.

Yup, biting your thumb was the lower-class insult at the time.

Ros was the girl that Romeo was infatuated with, she was going to be a nun or something, from memory.

ok thanks ;)

If you were a peasant type chap way back when, you couldn't verbally insult someone of higher station, so you might bite your thumb toward them, indicating that you are insulting them but can't say it. It became a common insult in Shakespeare.