Possible Clues:

• "How stupid of me!"
• Cry accompanying a head slap
• "Simpsons" word added to the OED
• First note of tonic sol-fa
• 'Why did I do that?!'
• "How dumb of me!"
• First note in sol-fa scale
• 'I'm so stupid!'
• Head-slapping word from Homer
• Self-critical cry