late 14c., from Greek hamadryas (plural hamadryades) "wood-nymph," fabled to die with her tree, from hama "together with" (see same) + dryas (plural dryades) "wood nymph," from drus (genitive dryos) "tree," especially "oak," from PIE *deru- "tree, wood, oak" (see tree (n.)). Use in 19c. biology for a type of butterfly, a type of venomous Indian serpent, and a type of large hairy baboon.