1In Biblical translation or allusion: an asp or other snake, regarded as deaf or wilfully unhearing. Compare "as deaf as an adder", adder-deaf .
2British dialect The slow-worm, Anguis fragilis.
3Any of certain (venomous or non-venomous) snakes, especially (a) Australian a death adder (genus Acanthophis); (b) US the copperhead, Agkistrodon contortrix (now rare).
Old English; earliest use found in The Vespasian Psalter. From deaf + adder, originally after post-classical Latin aspis surda (Vulgate: Roman Psalter, Gallican Psalter), itself after Hebrew p̄eṯen-hērēš (Psalm 58:4; from p̄eṯen viper, adder + hērēš deaf).