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"

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.