The great grandfather’s or great grandmother’s grandfather; a fourth grandfather. The ascending line of lineal ancestry runs thus: Pater, Avus, Proavus, Abavus, At avus, Tritavus. The seventh generation in the ascending scale will be Tritavi pater, and the next above it Proavi atavus.
Law Dictionary – Alternative Legal Definition
The male ascendant in the fifth degree, was so called among the Romans, and in tables of genealogy the term is still employed.