Related Clues:

• 'Don't think so!'
• Opposite of 15-Across
• Never!
• "Fat chance!"
• Aye undoer
• Scrapped, at NASA
• "Impossible!" and the theme of this puzzle
• "Don't think so"
• 'Unh-unh'
• "You gotta be kidding!"
• "I don't think so!"
• Adamant denial
• No, informally
• Denial
• What cynics say?