1Zoology and Medicine. Having only one digit, finger, or toe (as a normal condition in some mammals or a congenital abnormality in humans and other vertebrates).
2Zoology and Entomology. Having a single claw, especially one that is able to fold back on a leg in order to grasp.
Origin
Mid 19th century; earliest use found in Noah Webster (1758–1843), lexicographer. From mono- + -dactylous.