What is the origin of the idiom "be Greek to someone" and when to use it?

The idiom "be Greek to someone" originates from ancient Rome. In his play "Julius Caesar," written around 1599, William Shakespeare used a similar phrase, "It is Greek to me," to indicate that something is incomprehensible or difficult to understand. It is believed that Shakespeare's use of the phrase was inspired by the linguistic complexity of the Greek language for English speakers at the time. This expression is used when someone encounters something that is completely unintelligible or incomprehensible.