Strong's G2191 · Greek

ἔχιδνα

echidna · ekh'-id-nah

of uncertain origin; an adder or other poisonous snake (literally or figuratively)

Scholarly Lexicons

Abbott-Smith Manual Greek Lexicon

 5 ** ἔχιδνα , -ης, ἡ , [in OT (Aq.), Is 59:5 *;] a viper : Ac 28:3 ; metaph., γεννήματα ἐχιδνῶν , Mt 3:7 12:34 23:33 , Lk 3:7 .† 

G. Abbott-Smith, A Manual Greek Lexicon of the New Testament (1922) — public domain