Possible Clues:

• Word with a head slap
• 'I can't believe I missed that question!'
• Sound often heard after a slap
• "Like, no kidding!"
• 'I would have thought it was obvious'
• Response to the obvious
• 'I am so stupid!'
• "Like, no-brainer!"
• Forehead-slapper's syllable
• Dolt's response