1580s, "pertaining to the brackish sea between the Scandinavian peninsula and Eastern Europe," from Medieval Latin Balticus, perhaps from Lithuanian baltas "white" or Scandinavian balta "belt; strait" (in reference to its narrow entranceway). In German, it is Ostsee, literally "east sea." From 1887 as the name of a language group comprising Lithuanian, Lettish, and Old Prussian.
<a href="https://www.etymonline.com/word/balto-">Etymology of balto- by etymonline</a>
Harper, D. (n.d.). Etymology of balto-. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/balto-