This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

v. Present participle of underdo.

"Certainly if there were any fault in Mr Codlin's usual deportment, it was that he rather underdid his kindness to those about him, than overdid it."

overdo   

underdid    underdone