Possible Clues:

• Mild oath
• "For dang sure!"
• 'As sure as shootin' ...'
• Gosh! Sweet manufacturer's slogan when advertising on air
• Brit's emphatic phrase
• 'Dang!'
• “So help me!”
• It's emphatically uttered through part of one's mouth
• Crikey, so that's how envelopes are sealed?
• Adjuration of neighbouring tree