cock a snook

While the phrase "thumb one's nose" first appeared in English around 1903, "cocking a snook" is much older, first appearing in print back in 1791. The verb "to cock" comes from strutting behavior of male chickens, and means, as the Oxford English Dictionary puts it, "to turn up in an assertive, pretentious, jaunty, saucy, or defiant way." The "snook" is of uncertain origin, but may be related to "snout," which would certainly make sense.