late 14c., "one of the halves of a folding door," from Latin valva (plural valvae) "section of a folding or revolving door," literally "that which turns," related to volvere "to roll," from PIE root *wel- (3) "to turn, revolve." Sense extended 1610s to "membranous fold regulating flow of bodily fluids;" 1650s to "mechanical device that works like an anatomical valve;" and 1660s in zoology to "halves of a hinged shell." Related: Valved.
<a href="https://www.etymonline.com/word/univalve">Etymology of univalve by etymonline</a>
Harper, D. (n.d.). Etymology of univalve. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/univalve