A tall, cylindrical military hat or headdress, especially one worn by Russian soldiers or cavalry.
From Russian 'kolpak' (a type of hat or cap), borrowed into English military vocabulary in the 19th century when European armies adopted various ethnic military styles.
The colback became so iconic in military uniforms that modern ceremonial guards still wear similar tall hats—and nobody realizes they're wearing essentially the same thing Russian cavalry wore 200 years ago!
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