This is the 11184th most frequent Russian word.
греческом
"Греческом" means "Greek" (in the prepositional case).
Here, "греческом" is used to indicate the language in which the book was read, "Greek."
Мы читали книгу на греческом языке.
We read a book in the Greek language.
In this sentence, "греческом" qualifies the noun "острове" (island), describing it as Greek.
На греческом острове есть древние руины.
On the Greek island, there are ancient ruins.
Here, "греческом" describes the dress, indicating that its style or origin is Greek.