Wiktionary
thumb one's nose
vb. 1 To place a thumb upon the tip of the nose, typically with the fingers spread and while simultaneously wiggle one's fingers, in a gesture of disrespect. 2 (context figuratively English) To act disrespectfully, especially by flouting the object of disrespect.
Usage examples of "thumb one's nose".
When one is ten and has lived by one's wits in the streets since the age of five or six, it is easy enough to thumb one's nose at power.