un·sound / ˌənˈsound/ • adj. not safe or robust; in poor condition: the tower is structurally unsound. ∎ not based on sound evidence or reasoning and therefore unreliable or unacceptable: unsafe and unsound banking practices. ∎ (of a person) not competent, reliable, or holding acceptable views. ∎ injured, ill, or diseased, esp. (of a horse) lame.DERIVATIVES: un·sound·ly adv.un·sound·ness n.