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

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v. Third-person singular simple present indicative form of underwhelm.

"At a sales convention, he underwhelms his colleagues with puffed-up patter, dismisses the idea of giving his hardworking secretary a better job because of the limitations of the "female brain" and then displays his own astute powers of observation by picking up an overserved blonde who steals his wallet."

overwhelm   

underwhelmed    underwhelming    underwhelms   

Helm    elm    helm    overwhelm    realm