Learn That Word

Synonyms for Underestimate (same or very similar meaning)

WordNet sense 1 (an estimation that is too low; an estimate that is less than the true or actual value):
underestimation, underrating, underreckoning

WordNet sense 2 (assign too low a value to):

WordNet sense 3 (make a deliberately low estimate):

WordNet sense 4 (make too low an estimate of):

From the ODE community, based on WordNetadd/edit