“Cocking a snook” is a sign of derision or contempt, made by putting the thumb on the nose, holding the palm open and perpendicular to the face, and wiggling the remaining fingers. It is used mostly by schoolchildren, often combined with verbal insults, sticking out the tongue or blowing a raspberry. Americans call it the “the five-fingered salute.”
It is also known as thumbing the nose, Anne’s Fan or Queen Anne’s Fan.
The Oxford English Dictionary cites a figurative 1938 usage: “The Rome–Berlin axis… cocked the biggest snook yet at the League of Nations idea” by Eric Ambler in his Cause for Alarm.
We have no idea where this odd phrase came from.
The first written reference to the phrase appears to be from Wynne’s Diary, 1791:
“They cock snooks at one on every occasion.”
A much later one is in The Times, 1904: “The young monkey puts his tongue in his cheek and cocks a snook at you.”
Since we don’t know where the phrase originated, it’s a guessing game that doesn’t have an answer. The word “snook” seems to mean the nose, but the only official use of “snook” is as the name of a fish. The word “cock” has several meanings, but in this case I would compare it to cocking a gun. You could say that you’re cocking your nose to fire insult at someone.
My father used to cock his snook sometimes, but he called it “thumbing my nose,” and to him it meant “kiss my ass.” And that meant, “Don’t be ridiculous.” Or, as today’s slang would have it, “As if!”
Rainforest Writer 
Interesting. I don’t actually remember seeing anyone do this recently, and I didn’t associate it with the term; always thought of it as nose-thumbing.
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