serpentine n.

c. 1300, "limbless reptile," also the tempter in Genesis iii.1-5, from Old French serpent, sarpent "snake, serpent" (12c.), from Latin serpentem (nominative serpens) "snake; creeping thing," also the name of a constellation, from present participle of serpere "to creep."

This is reconstructed to be from PIE *serp- "to crawl, creep" (source also of Sanskrit sarpati "creeps," sarpah "serpent;" Greek herpein "to creep," herpeton "serpent;" Albanian garper "serpent").

Serpent and snake now mean precisely the same thing ; but the word serpent is somewhat more formal or technical than snake, so that it seldom applies to the limbless lizards, many of which are popularly mistaken for and called snakes, and snake had originally a specific meaning. [Century Dictionary, 1902]

Used figuratively of things spiral or regularly sinuous, such as a type of musical instrument with a twisting tube (1730). Serpent's tongue as figurative of venomous or stinging speech is from mistaken medieval notion that the serpent's tongue was its "sting." Serpent's tongue also was a name given to fossil shark's teeth (c. 1600). Serpent-charmer is by 1861.

also -in, adjectival word-forming element, Middle English, from Old French -in/-ine, or directly from Latin suffix -inus/-ina/-inum "of, like," forming adjectives and derived nouns, as in divinus, feminus, caninus; from PIE adjectival suffix *-no- (see -en (2)).

The Latin suffix is cognate with Greek -inos/-ine/-inon, and in some modern scientific words the element is from Greek. Added to names, it meant "of or pertaining to, of the nature of" (Florentinus), and so it also was commonly used in forming Roman proper names, originally appellatives (Augustinus, Constantinus, Justinus, etc.) and its descendants in Romanic languages continued active in name-forming. The Latin fem. form, -ina, was used in forming abstracts (doctrina, medicina). Relics of the attempt to continue a distinction between Latin -ina and -inus account for the English hesitation in spelling between -in and -ine.

before vowels ophi-, word-forming element meaning "a snake, serpent," from Greek ophio-, combining form of ophis "serpent, a snake," probably from PIE *egwhi- (source also of Sanskrit ahi-, Avestan aži- "snake," and perhaps the first element of a Germanic word for "lizard," i.e. Old High German egi-dehsa). Hence ophiolatry "serpent-worship" (1862), and the 2c. sect of the Ophitæ, who revered the serpent as the symbol of divine wisdom.