Gärtner observes that from Plato onwards, ἁπλότης is “compounded from ha-, together, and pel-, to fold” and means “singleness, single; the opposite being diplous, double. Beside this numerical meaning there grew up a positive ethical connotation for the word-groups. Hence, haplotēs [ἁπλότης] came to mean … straightness, openness, speaking without a hidden meaning
Ephesians, An exegetical commentary