To unsoundness of mind is shown; there was certainly a degree of debility from age and supervening infirmities, the understanding was weakened and impaired, but that does not render a person intestable or incapable of judging of injuries done to him or of benefits confered upon him.
-- The Law Times, Volume 69, 1880
I understood that a man convicted by the verdict of his country of housebreaking is infamous and intestable.
-- James Boswell, Mark Harris, The heart of Boswell: six journals in one volume
Intestable shares the same root as testify, the Latin testā, "to bear witness."