serpent (n.) Look up serpent at Dictionary.com
c.1300, "limbless reptile," also the tempter in Gen. 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," from PIE *serp- "to crawl, creep" (cognates: Sanskrit sarpati "creeps," sarpah "serpent;" Greek herpein "to creep," herpeton "serpent;" Albanian garper "serpent").

Used figuratively of things spiral or regularly sinuous, such as a type of musical instrument (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).