"beyond or different from what is natural," 1570s, from Medieval Latin preternaturalis (mid-13c.), from Latin phrase praeter naturam (praeterque fatum) "beyond nature (and beyond fate)," from praeter "beyond, over, more than in quantity or degree" (see preter-) + accusative of natura "nature" (see natural (adj.)).
Used at least since 1770s in the sense of supernatural, but technically and properly distinct from that word. "Preternatural is used especially to note that which might have been a work of nature, but is not" [Century Dictionary]. Related: Preternaturally; preternaturalness; preternaturalism.