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Some sources claim that the last ethnic Prussin was burnt on the stake in the mid 14 century
The articles in Britannica regarding Prussia and Baltic languages state that Prussian was spoken until 17th century. The wiki article says that:
Old Prussian probably ceased to be spoken around the end of the 17th century with the Great Plague.
I see no reason for the German colonists to speak the language of the inhabitants, so it seems that a Prussian ethnicity survived well until the 17th century, anbeit by their language.
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