Horse 1471 - I Bite My Thumb At You

Whilst walking to the railway station; after getting off a bus, I got caught on a traffic island and was standing patiently for the little green man to show, when a stretched limousine came through the intersection; honking and flashing its lights. Clearly the driver was both very angry indeed and in a hurry.
In turn this made me quite cross indeed and so rather than give him the one fingered salute, I did that most quaint of things... I bit my thumb at him.

Gasp! Shock! Horror!...other things with too man exclamation points!

Of course to a modern audience, biting one's thumb really only comes to use via the opening scene in Romeo and Juliet. Sampson of the house of Capulet even pauses to explain what it meant., sort of:

Sampson:

Nay, as they dare. I will bite my thumb at them;
which is a disgrace to them, if they bear it.
- Scene 1, Act 1, Romeo and Juliet

Apart from rumour and wild mass guessing which is what the internet abounds in, you won't really find much to explain what it's about. There is a similar gesture in modern Sicily in which the thumb is placed behind the incisors and flung forward, which still conveys a great deal of venom but it's worth remembering that not only had Shakespeare never been to Italy, that Sicily is more than 1100km from Verona where the play is set, but that in 1597, Italy wasn't a unified kingdom and barely had a unified language; even now Italy is less homogeneous than other European nations.

Curiously, I found an entirely satisfactory explanation of what biting one's thumb meant, not in the works of any Italian but that "most distinguished man of letters in English history", none other than Dr Samuel Johnson. He recounts that biting one's thumb is not about being rude at all but precisely the opposite.
The act of biting one's thumb prevents one from speaking any words offensive or otherwise; thus biting one's thumb is actually a gesture more along the lines of:
"I have something despicable to say to you, and about you, but I shall not because it would cause an offence to hear it".
In the grander context of Romeo and Juliet, which is a play as much about mixed messages and the misinterpretation of them, such an act not only fits perfectly within context but in the wider society of Elizabethan England.

I personally like the act of biting one's thumb precisely because it is archaic. People think that it is highly offensive even though they have no idea why, yet in a weird sort of way because it is so well known to have appeared in "high" literature, it almost makes you stop and think "well played, sire"... and because simply flipping the bird at someone is vulgar.