From an unused root meaning to twist; an asp (from its contortions) -- adder.
Englishman's Concordance
1) a snake, venomous serpent 1a) perhaps the cobra, adder, or viper Part of Speech: noun masculine A Related Word by BDB/Strong’s Number: from an unused root meaning to twist Same Word by TWOT Number: 1858a
Deuteronomy 32:33 HEB:יֵינָ֑ם וְרֹ֥אשׁ פְּתָנִ֖ים אַכְזָֽר׃ NAS: And the deadly poison of cobras. KJV: and the cruel venom of asps. INT: their wine poison of cobras and the deadly
Job 20:14 HEB:נֶהְפָּ֑ךְ מְרוֹרַ֖ת פְּתָנִ֣ים בְּקִרְבּֽוֹ׃ NAS: To the venom of cobras within KJV: [it is] the gall of asps within INT: is changed to the venom of cobras within
Job 20:16 HEB: רֹאשׁ־ פְּתָנִ֥ים יִינָ֑ק תַּֽ֝הַרְגֵ֗הוּ NAS: the poison of cobras; The viper's KJV: the poison of asps: the viper's INT: the poison of cobras sucks slays
Psalm 58:4 HEB:נָחָ֑שׁ כְּמוֹ־ פֶ֥תֶן חֵ֝רֵ֗שׁ יַאְטֵ֥ם NAS: a deaf cobra that stops KJV: [they are] like the deaf adder [that] stoppeth INT: of a serpent Like cobra A deaf stops
Psalm 91:13 HEB:עַל־ שַׁ֣חַל וָפֶ֣תֶן תִּדְרֹ֑ךְ תִּרְמֹ֖ס NAS: upon the lion and cobra, The young lion KJV: upon the lion and adder: the young lion INT: upon the lion and cobra will tread will trample
Isaiah 11:8 HEB:עַל־ חֻ֣ר פָּ֑תֶן וְעַל֙ מְאוּרַ֣ת NAS: by the hole of the cobra, And the weaned KJV: on the hole of the asp, and the weaned child INT: and cave of the cobra and den
, whence perhaps Arabic 
; — only in poetry: plural מְּתָנִים ראֹשׁDeuteronomy 32:33 ("" חֲמַת תַּנִּינִם), Job 20:16, ׳מְרוֺרַת פJob 20:14; singular מֶּ֫תֶןPsalm 91:13 ("" תַּנִּין), חֵרֵשׁ׳פPsalm 58:5, חֻר מָּ֑תֶןIsaiah 11:8.