1. (noun)underestimate, underestimation, underrating, underreckoning an estimation that is too low; an estimate that is less than the true or actual value
2. (verb)undervalue, underestimate assign too low a value to
"Don't underestimate the value of this heirloom-you may sell it at a good price"
3. (verb)lowball, underestimate make a deliberately low estimate
"The construction company wanted the contract badly and lowballed"
4. (verb)underestimate, underrate make too low an estimate of
"he underestimated the work that went into the renovation"; "Don't underestimate the danger of such a raft trip on this river"