To "flatter oneself" means to believe something positive, proud, or pleasing about oneself that is usually untrue, exaggerated, or unsupported by others. It involves overestimating one's own abilities, achievements, or importance. It is commonly used in the negative phrase "don't flatter yourself" to tell someone not to be conceited or to stop assuming they are special. 

Key Aspects:

• Self-Deception: It involves choosing to believe a better version of reality.
• Usage: Frequently used to imply someone is delusional about their skills or importance, e.g., "I flatter myself that I'm a great singer" (implying I might not be).
• "Don't flatter yourself": A common idiom used to tell someone to stop acting arrogant or overestimating their importance in a situation. 

Example Sentence: "She flattered herself that she was the best dancer in the room, when in reality, she was quite average."