
Cock a snook at somebody/something
Cultural Nuance
Primarily British and considered informal, so use it in casual settings, not in formal or professional conversations. 
Visual 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. 
Disrespect Shown
Indicates a deliberate and open lack of respect, often done in a mocking or cheeky manner. 