Estimation of the maximum horizontal in situ stress from logged borehole-breakout dimensions in brittle elastic rocks requires knowledge of the true triaxial compressive strength of the medium. We employed our true triaxial loading apparatus to determine the strength criterion of the nearly impermeable amphibolite penetrated by the German Continental Deep Drilling Program (KTB) superdeep scientific hole at depths of 3200--7800 m. To better simulate borehole wall conditions, we left one pair of the prismatic specimens' faces unjacketed and in direct contact with the confining fluid. This testing variation brings about a fundamentally different failure mechanism from that in fully jacketed (dry) specimens. Brittle fracture occurs at or soon after dilatancy onset and results from the development of densely spaced extensile fractures subparallel and adjacent to one of the unjacketed faces. We infer that the confining fluid intrudes newly opened microcracks, which are predominantly subparallel to the unjacketed faces, and promotes their elongation into throughgoing fractures. For any given least principal stress, the compressive strength typically increases with the rise in the intermediate principal stress. The true triaxial strength criterion of the unjacketed amphibolite can be expressed as a linear relationship between the octahedral shear stress and the octahedral normal stress at failure. Employing this criterion together with all the other known data from the KTB hole, we recomputed the magnitude of the maximum horizontal in situ stress there. Our results show that it increases steadily with depth, with a relatively narrow band of uncertainty, confirming previous assessments of a strike-slip stress regime.