It is completely unintelligible; I don’t understand. This term, used by generations of schoolchildren, was coined by Shakespeare in Julius Caesar (1.2), where the conspirator Casca says of Cicero’s speech, “For mine own part, it was Greek to me.” In the play Cicero actually spoke in Greek, in order to prevent some people from understanding, but the term soon was transferred to anything unintelligible and has been so used ever since.