Choose the correct meaning of idiom:

The idiom "to bite one's thumb at someone" means to insult or show contempt toward someone in a disrespectful or defiant manner. The expression originated as a rude gesture used to provoke or offend another person.

Therefore, the correct answer is To insult someone contemptuously.

• To mock one's own pride – Incorrect. The idiom is not about mocking oneself.
• To insult someone contemptuously – Correct Answer. It means to show open disrespect or contempt toward someone.
• To offer false flattery – Incorrect. This refers to insincere praise, not an insult.
• To express internal fear – Incorrect. The idiom has no connection with fear.