Cock a snook at somebody/something
Definition 1 of 1
Pro Tip 1/3Cultural Nuance
Primarily British and considered informal, so use it in casual settings, not in formal or professional conversations. 
At the pub, Joe cocked a snook at the new regulations.
Pro Tip 2/3Visual Gesture
Originates from a rude gesture where a thumb is placed on the nose with fingers spread and wiggled. Understanding this can help remember its meaning. 
Mimicking the gesture, she cocked a snook at the absurd comment.
Pro Tip 3/3Disrespect Shown
Indicates a deliberate and open lack of respect, often done in a mocking or cheeky manner. 
The rebellious teenager cocked a snook at the school's dress code.