The word deciduous was first attested in the 1680s as an adjective used to refer to anything that eventually falls off or descends with time. This was used to apply to subjects as varied as teeth, shooting stars, and testicles, but eventually it evolved into just describing plants which have leaves that fall off. Deciduous was taken from Latin deciduus, a word that meant "that which falls down" and developed from decidere, "to fall off" or "to fall down". Moving back in time, we can eliminate the prefix de-, which here indicated a downwardly direction but could also mean "off" or "from" (tracing to PIE de, same definition). The remaining verb is cadere, meaning "to fall", and that, through Proto-Italic kado, derives from the Proto-Indo-European reconstruction khd, also "fall".
Hello! I'm Adam Aleksic, a sophomore studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy politics, vexillology, geography, board games, conlanging, and law.