The dictionaries seem to attribute the words to Latin, while the -oo sound seems to come from Greek. The root word of both coming from a word for fold, "-plus" in Latin and "-plos" or "-ploos" in Greek.
Greek and Latin have been mixed up enough over the years that it should be acceptable to use either pronunciation regardless of the true origin of the word.
Greek and Latin have been mixed up enough over the years that it should be acceptable to use either pronunciation regardless of the true origin of the word.