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):
undervalue
WordNet sense 3 (make a deliberately low estimate):
lowball
WordNet sense 4 (make too low an estimate of):
underrate
From the ODE community, based on WordNet
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