Used figuratively to refer to the French Foreign Office. It might also be used to refer to the French government in general, particularly in relation to foreign affairs. Pronounced kay-dor-say.
The Quai d'Orsay is a street in Paris, parallelling the southern bank of the Seine River (In French, quai is applied to a street along the waterfront), extending from the Eiffel Tower to the Palais Bourbon. Right next to the Palais Bourbon stands the French ministry of foreign affairs. Hence the metonymy.