an estimation that is too low; an estimate that is less than the true or actual value
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"
with diligence; in a diligent manner; "we may diligently observe the Lord's supper on the first day of the week, diligently preach the gospel, or minister to the saint"