Definitions:

1. An exclamation used to express frustration, realization of a mistake, or disappointment. interjection
   • I let out a loud "D'oh!" after I realized my mistake.
   • When the teacher pointed out the simple mistake in his equation, the student couldn't help but exclaim, 'D'oh!'.
2. Used to express sudden recognition of a foolish blunder or an ironic turn of events. INTERJECTION
   • Realizing he had missed the last step in the recipe, he slapped his forehead and said, 'D'oh!.

Synonyms:

• "ah, shoot!"
• "argh!"
• "damn!"
• "my bad."
• "oh no!"
• "oh, snap!"
• "oops!"
• "ugh!"
• "whoops!"
• "yikes!"