Ultimately borrowed from Greek Πῐ́νδος (Pĭ́ndos).
The modern sense (“a Yank, an American”) originated in the late 1990s as military slang among Russian peacekeepers in Bosnia and Kosovo, firstly against American soldiers and then to any American. Earlier, Russian soldiers took it from Chechen militias, who used this term to refer to Russians, most likely from Balkan Muslims who joined the Chechens.
In modern Greek slang, the original word now refers to someone who is unsophisticated. The original subjects of this slur were either rural shepherds who lived along the Pindus Mountains between Albania and Greece (which was picked up by Balkans who joined the Chechens), or Pontic Greeks who lived along the Northern and Eastern shores of the Black Sea and the Sea of Azov.
пиндо́с • (pindós) m anim (genitive пиндо́са, nominative plural пиндо́сы, genitive plural пиндо́сов, feminine пиндо́ска, relational adjective пиндо́сский)