Thumb one's nose at

To thumb one's nose at someone or something 

1. Lit. to show a sign of derision at someone or something by placing the thumb to the side of the nose. (Often while wiggling the other fingers of the hand.)

• Don't thumb your nose at me unless you want a fight
• Fred thumbed his nose at the car as it drove off.

2. Fig. to dismiss someone or something as worthless, verbally. 

• Walter thumbed his nose at Fred and asked the gang to send someone else to do the job. 
• She thumbed her nose at the whole idea.