Possible Clues:

• 'That's so obvious!'
• Cry with a head slap
• 'No shit, Sherlock'
• [Forehead slap]
• ''Boy, am I stupid!''
• "Obvi"
• "What an idiot I am!"
• "How could I miss that?!"
• "Okay, I get it already"
• Brow-slapper's syllable