From New Latin Prūssia, a Latinization used by Peter of Dusburg of a Baltic (Old Prussian, or perhaps Lithuanian or Latvian) autonym. The Proto-Indo-European source of the name is unclear; more at Prussia. Compare the Proto-Balto-Slavic *prus-sk-, whose cognates include Proto-Slavic *prъskati (“to splutter, to splash”), Sanskrit प्रुष्णोति (pruṣṇóti, “to sprinkle”), and thus signifying "watery land". The Middle English designation for the region, Pruce, derives from the same Latinization and is the source of the terms pruce and spruce.

Etymology: Wiktionary (CC-BY-SA)