We predict that you will guess the correct origins of retrodict, and chances are we will not contradict you. English speakers had started using predict by at least the late 16th century; it's a word formed by combining prae- (meaning "before") and dicere (meaning "to say"). Since the rough translation of predict is "to say before," it's no surprise that when people in the early 20th century wanted a word for "predicting" the past, they created it by combining the prefix for "backward" (retro-) with the -dict of predict. Other dicere descendants in English include contradict, benediction, dictate, diction, and dictionary.