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"
having no cause or apparent cause; "a causeless miracle"; "fortuitous encounters--strange accidents of fortune"; "we cannot regard artistic invention as...uncaused and unrelated to the times"
having no justifying cause or reason; "a senseless, causeless murder"; "a causeless war that never had an aim"; "an apparently arbitrary and reasonless change"