Possible Clues:

• Hit-in-the-solar-plexus noise
• "Pow!" response
• Sock-in-the-stomach sound
• Response to 'pow!' in cartoons
• Cartoon cry
• "Ouch!" kin
• Reaction to a slug?
• Response to a stomach punch
• Old rhino which is stepped on by 'orse?
• ''Ouch!'' relative