The Greek noun echidna (ἔχιδνα) means viper, poisonous snake. It appears five times in the NT: three times in the phrase 'brood of vipers' (gennēmata echidnōn) used by John the Baptist (Matt 3:7; Luke 3:7) and Jesus (Matt 12:34; 23:33), and once in the account of Paul's snakebite on Malta (Acts 28:3).
The phrase 'brood of vipers' (gennēmata echidnōn) is among the most severe denunciations in the NT. John the Baptist leveled it at the Pharisees and Sadducees who came to his baptism (Matt 3:7), and Jesus used it against the Pharisees (Matt 12:34) and the scribes and Pharisees collectively (Matt 23:33). The imagery implies not just individual toxicity but generational wickedness — the offspring of vipers produce more vipers. The literal echidna in Acts 28:3–6, which bit Paul and did him no harm, fulfilled Jesus' promise that his disciples would 'pick up serpents' without injury (Mark 16:18).