Sadahide worked in Bungo Province at the turn of the and periods, and the published sources record him as a monk of Mount Hiko, the old religious mountain of Kyushu. They place him at the root of the Bungo tradition beside its founding figure, Yukihira, though they cannot settle which of the two came first: "Sadahide was a monk of Hikosan, said to be either the teacher or the disciple of Yukihira" (定秀は彦山の僧で行平の師とも弟子ともいう). No dated blade survives to fix him in time. What survives instead is a small group of signed inscribed Bungo no so Sadahide, and the form of those blades does the dating: slender, with a high -zori and clear , the point sinking a little to a , an archaic and elegant shape that, in the words of the 70th commentary, does not descend below the early .
His hand is read first in the calm of the temper. Over the whole of his signed work runs a narrow carrying a shallow , into which he mixes a quiet rather than the showy clove-flower of contemporary . The tell of that edge is its , which the published sources describe again and again as softly clouded, , never bright and clear; the appraisal of his puts it that the temper is -toned with small irregularities, the "moist and softened" (匂口がうるんで). Thick gathers along it, and -like crescents, a faint , fine and work through the edge without ever crowding it. The runs straight to a on one face and turns in a small on the other, often with . It is, as the commentary on the signed ko- has it, the "gentle, restrained charm of a Kyushu work" (九州物らしい穏やかな魅力).
The carries the archaic register. He forges an that flows toward and stands a little, well refined in the manner the sources liken to Yukihira's own, with adhering and fine entering. Over it the steel takes a whitish, cast rather than the bright of ; the prewar Bijutsuhin note only an -like effect on the . Taken together, the flowing whitish and the -laden, clouded are exactly what the 70th entry calls the "characteristic and of the Kyushu classical group" (九州古典派の地刃の特徴が表れている), and they are the surest way his work is told from the brighter classical hands of and Yamashiro.
There are two faces to his record. The first is the signed and ko- just described, the recognized prime of his hand. The second is the in with that Mitsutsune appraised as Sadahide in 1678, issuing a thirty-sheet and cutting the name in vermilion. On that blade the is in nothing different from his usual, but the turns especially exuberantly in the upper half, an effect, the commentary states plainly, "not seen in Yukihira" (行平には見られない出来である). The carries a raised-relief and the a , carvings of a kind also found on Yukihira. Mitsutsune judged the blade an early Bungo work and, precisely because it showed a workmanship Yukihira does not, named it Sadahide, a judgment the published sources call admirable.
What sets Sadahide apart within his own circle is named in the sources themselves. He is held close to Yukihira in his and his -laden , yet distinguished from him by that upper-blade and by the scale and manner of his signature, which the judges admit is itself unsettled: "the signatures of Sadahide seen to date are not uniform, and there is room for study as to which should be taken as typical" (経眼した定秀の銘振りは一律ではなく). One cuts the character katsu together with cherry blossoms, a motif also seen on Yukihira, while a carved katsu is otherwise known on Ohara Sanemori of old Hoki, so the connoisseurship places him squarely among the classical hands of the western provinces. He stands before the flowering of Bungo into its later, more numerous schools, one of the quiet early roots of swordmaking on the southern island.
For the collector he is a rare and early name. Fujishiro grades him Jo-jo . He has no National Treasures and no Important Cultural Properties on record; his standing rests instead on the prewar Bijutsuhin designations of three signed and on two modern blades, the signed ko- of the 70th session and the -appraised of the 12th. The published commentary makes the point that, signed Sadahide being so few, "the very signature of Sadahide, of which in- examples are extremely scarce, is exceptionally valuable" (在銘品が極めて少ない定秀の銘字も非常に貴重). His blades are documents as much as swords. One of the Bijutsuhin is transmitted as a piece "received by Mori Hidechika, the first Mori lord, from Emperor Go-Mizunoo" (毛利家初代秀就が後水尾天皇より拝領したものと伝える), and the Mori house and the Tokyo collector Takenaka Jiro appear among his recorded owners; of present whereabouts the record names the Kyushu National Museum, Iyo Inari Jinja and Itsukushima Jinja. A signed Sadahide comes to light only seldom, and a privately held example would be among the rarer things a collector of early Kyushu work could hope to encounter, valued less for availability than for the document it carries of how the Bungo tradition began.