underpromise

under- +‎ promise

underpromise (third-person singular simple present underpromises, present participle underpromising, simple past and past participle underpromised)

1. (intransitive) To promise less than one expects to do.
   • 2008 April 4, Richard Sandomir, “Fixing a Broken Dream One Night at a Time”, in New York Times‎:

     “He’s wise not to overpromise, but to underpromise and overdeliver,” said Dave Checketts, a former president of the Knicks and then Madison Square Garden who left in 2001 after a power struggle with James L. Dolan, the chairman of the Garden.