Emily Dickinson Lexicon

CO-E-TA'NE-OUS, a. [L. coætaneus; con and ætas, age. Coetanean is rarely used.]

Of the same age with another; beginning to exist at the same time; with to. “Every fault has penal effects coetaneous to the act.” But with may be preferable to to. This word is sometimes used as synonymous with cotemporary; but coetaneous seems properly to denote cotemporary in origin, rather than cotemporary in existence at any other period. It may however be used in both senses.