Related Clues:

• With 39-Across and 49-Across, rule broken by the three theme entries' first and last two letters
• 'On second thought, forget it'
• Word seen on both sides of ''say''
• A gruff no
• Opposite of ''yeah''
• 'Prob'ly not'
• Under no circumstances
• 'Hmm, don't think so'
• "Not gonna happen!"
• 'You're mistaken'
• 'None for me'
• 'No siree!'
• 'Nah'
• 'Sorry, dude'
• No, nastily