From an unused root meaning to twist; an asp (from its contortions) -- adder.
H6620. pethen
מֶּ֫תֶן noun masculinePsalm 68:5 a venomous serpent, perhaps cobra (פתן Ecclus 39:30; Aramaic מִּתְנָא, , whence perhaps Arabic ; — only in poetry: plural ראֹשׁ מְּתָנִים Deuteronomy 32:33 ("" תַּנִּינִם חֲמַת), Job 20:16, פ ׳מְרוֺרַת 20:14; singular מֶּ֫תֶן Psalm 91:13 ("" תַּנִּין), ׳פ חֵרֵשׁ 58:5, מָּ֑תֶן חֻר Isaiah 11:8.