tourde force

tour de force (to͞or' də fôrsʹ, fōrsʹ) n. pl. tours de force (to͞or')

A feat requiring great virtuosity or strength, often deliberately undertaken for its difficulty: “In an extraordinary structural tour de force the novel maintains a dual focus” (Julian Moynahan).

  [French : tour, turn, feat + de, of + force, strength.]

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