pre-Greek substrate

Ancient Greek has an estimated 1,000+ words that don’t appear to be of Indo-European origin, and thus are believed to have been borrowed from the languages that were spoken in Greece before Indo-Europeans migrated there. The words encompass a range of geographical, botanical and cultural topics (among others), as well as the names of many Greek gods.

In many cases, one of the leading pieces of evidence for why these words cannot be Indo-European is the high degree of variance between dialects. For example, even if the word ἀστεροπή asteropḗ “lightning” could be interpreted as “star-eye” (from ἀστήρ astḗr “star” + ὄψ óps “eye”) in the “standard” Attic dialect, there are a huge number of variant forms in other dialects that cannot be interpreted as “star” + “eye”. This makes it more likely that the reason for the variation is that different Greek dialects borrowed words from different pre-Greek languages/dialects.

The most important academic work currently out there talking about this topic is Robert S. P. Beekes’ Pre-Greek Phonology, Morphology, Lexicon (2014). I’m guessing that’s where the Reddit thread, which I have actually read, got a lot of its information from.

See Also / References