underestimate has 6 distinct synonyms spread across 4 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is overestimate.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
assign too low a value to
“Don't underestimate the value of this heirloom-you may sell it at a good price”
make too low an estimate of
“he underestimated the work that went into the renovation”
an estimation that is too low; an estimate that is less than the true or actual value
Synonyms, antonyms and the broader and narrower relations for underestimate come from Princeton University’s WordNet. Full sources and licences.