Prussian blue (Chem.), any one of several complex double cyanides of ferrous and ferric iron; specifically, a dark blue amorphous substance having a coppery luster, obtained by adding a solution of potassium ferrocyanide (yellow prussiate of potash) to a ferric salt. It is used in dyeing, in ink, etc. Called also Williamson's blue, insoluble Prussian blue, Berlin blue, etc. -- Prussian carp (Zoöl.) See Gibel. -- Prussian green. (Chem.) Same as Berlin green, under Berlin.

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OXFORD DICTIONARY

prussian, adj. & n.
--adj. of or relating to Prussia, a former German State, or relating to its rigidly militaristic tradition.
--n. a native of Prussia.

Idiom

Old Prussian the language spoken in Prussia until the 17th c. Prussian blue a deep blue pigment, ferric ferrocyanide, used in painting and dyeing.

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